Flow cytometric measurement of total DNA and incorporated halodeoxyuridine
The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California, for the operation of the Lawrence Livermore National Laboratory. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to flow cytometry. More specifically, it relates to the simultaneous measurement of incorporated halodeoxyuridine (HdU) and the total cellular DNA content by flow cytometric techniques. 
     The growth of any tissue at the cellular level is governed by variations in (1) the length of the cell cycle in the fraction of cells that continuously divide, (2) the growth fraction or the fraction of cells in the cell cycle at a given time and (3) the rate of cell loss. All these three mechanisms are important and operative in the growth of normal and malignant cells. A study of the cytokinetic properties of normal and tumor tissues, therefore, aid in the development of effective anticancer strategies for cell-cycle-phase-specific agents. 
     Cells in the body of an adult animal may be divided into three major categories: (1) differentiated cells i.e., cells which do not divide and proliferate and which eventually die; (2) cells that do not normally divide and therefore do not synthesize DNA but can be induced to do so by an appropriate stimulus; and (3) continuously dividing cells which actively synthesize DNA and which move around the cell cycle from one mitosis to the next. The life cycle of a continuously dividing cell exhibits three distinct and consecutive phases. There is a period, the G 1  phase, during which the cell is preparing for DNA synthesis but no actual, appreciable synthesis occurs. This phase is followed by the S phase, during which DNA is actively synthesized. During this phase, the cells take up nucleic acids present in the surrounding medium or environment, incorporating them into the DNA that is synthesized. The DNA content of the cell is almost doubled during the S phase. Following the S phase is the G 2  M phase during which period the cell prepares for mitosis. No DNA synthesis occurs in the G 2  M phase either. The synthesis of DNA is maximal in the mid-S phase. 
     The ability to measure, distinguish and differentiate between cell populations in the above three phases is, therefore, essential for many biological and biomedical investigations. The cell cycle traverse characteristics of normal and malignant cells, measured as a function of the frequency of DNA synthesizing cells, yield valuable information concerning the treatment prognosis and therapy for various types of cancer. 
     S. C. Barranco et al., Cancer Res., 42, 2894 (1982), studied the cell kinetics properties of CHO cells to direct the timing of two-drug combination treatment of normal and tumor cells. 
     M. G. Pallavicini et al., Cancer Res., 42, 3125 (1982), measured the DNA distribution sequences, tritiated thymidine uptake by tumor cells and the radioactivity of cells labeled with tritiated thymidine prior to and after treatment with 1-beta-D-arabinofuranosylcytosine (araC), a cytotoxic agent. 
     J. S. Hart et al., Cancer, 39, 1603 (1977), studied the prognostic significance of individual patient characteristics using eight pretreatment variables and reported that cytokinetic factors were important in the administration of chemotherapy and were of clinical importance in selecting approaches to therapy. 
     The cell fraction in the S phase may be determined by measuring the uptake of nucleic acids present in the environment. One of the most commonly used methods is the measurement of the uptake of labeled nucleic acids, such as thymidine, uridine or bromodeoxyuridine which is a chemical analog of thymidine and which is readily incorporated by the cell into its DNA. Flow cytometry has been applied for the measurement of DNA in cultured cells to monitor ploidy of cells and changes in cell growth pattern (M. A. Van Dilla et al, Science, 163, 1213 (1968)). 
     H. Quastler and F. G. Sherman, Exp. Cell Res., 17, 420 (1959) reported an analysis of the kinetics of a particular cell population, the different compartments existing in the system, transitions between the compartments which occur regularly, and estimates of the system parameters and individual variations. Their studies indicated that differentiation of the cells in their system occurred only during certain critical phases of the generative cycle. 
     J. W. Gray et al., in Cell Tissue Kinet., 10, 97 (1977) described a rapid method for the analysis of cell cycle traverse. According to this method, cells in S phase were pulse labeled with a radioactive DNA precursor and the progress of the labeled cells through the cell cycle was monitored by means of the label using flow cytometry and scintillation counting. 
     Recently, a method for the detection of BrdU incorporation using polyclonal antibodies has been reported by H. G. Gratzner et al., Exp. Cell Res., 95, 88 (1975); H. G. Gratzner et al., J. Histochem. Cytochem., 24, 34 (1976); H. G. Gratzner et al., Res. Comm. Chem. Pathol. Pharmacol., 20, 34 (1978). 
     Several measurement techniques, such as autoradiography, liquid scintillation counting, cytometric imaging, flow cytometry and the like, which have been developed over the years, suffer from serious limitations. Autoradiography is time consuming and involves laborious measurements. Furthermore, the method is also limited by the difficulty in distinguishing between unlabeled and weakly labeled cells and with background noise. 
     Scintillation counting which is based on the radiation emissions from the radioactive label, such as tritium,  14  C,  32  P and the like, requires measurements involving a small fraction of labeled cells in large cell populations and does not lend itself to single cell analysis or measurements. The use of antibodies offers an attractive alternative, but the polyclonal antibodies used exhibit variable specificity depending on the animal used for the immunization and also lack sufficient specificity. Furthermore, they cross-react with thymidine and are difficult to separate from the other immunoglobulins present in the system. 
     For the measurement of cell growth patterns, it is necessary to resolve the DNA distribution into its component parts. This analysis is not simple since the population distribution curves for the various phases of the cell cycle are, most often, quite complex due to considerable overlap in the measured DNA distribution between phases and due to inherent shortcomings in the mathematicals models, methods and techniques used. 
     It would, therefore, be desirable to have a relatively simple technique for completely resolving the DNA distributions in the three major phases of the cell cycle, G 1 , S and G 2  M. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the subject invention to provide a simple method for resolving cellular DNA distribution in the three major phases of the cell cycle. 
     It is another object of the present invention to measure the level of incorporation of DNA precursors as a measure of DNA synthesis during the various phases of the cell cycle. 
     It is a further object of the instant invention to provide a flow cytometric technique for the simultaneous measurement of the total DNA content of a cell and the level of DNA synthesis during the various phases of the cell cycle. 
     It is an additional object of the present invention to provide a novel use for monoclonal antibodies reactive with DNA precursors in the measurement of DNA synthesis during various phases of the cell cycle along with a simultaneous measurement of the total DNA content of the cell. 
     Yet another object of the present invention is to provide a method for the measurement of DNA synthesis during and following drug treatment in cancer therapy. 
     Still another object is to provide a method for the measurement of the rate of cell cycle traverse. 
     Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
     To achieve the foregoing and other objects and in accordance with the purpose of the present invention as embodied and broadly described herein, the subject invention is directed to a method for the simultaneous flow cytometric measurement of the total cellular DNA content and the amount of the uptake of DNA precursors as a measure of DNA synthesis during various phases of the cell cycle in normal and malignant cells in vitro and in vivo. A suitable DNA stain or a fluorescent dye which binds to cellular DNA, such as propidium iodide (PrdI), serves as a probe for the measurement of the total cellular DNA content while a monoclonal antibody reactive with the DNA precursor of choice serves as a probe for the newly synthesized DNA. The DNA precursor normally utilized in these measurements is a halogenated uridine (HdU), preferably bromo-, chloro- or iododeoxyuridine (BrdU, CldU, IdU), which is a chemical analog of thymidine and which is readily taken up by the cells and incorporated into their DNA. Basically, the method for the simultaneous measurement of the total cellular DNA content and of incorporated HdU comprises reacting cells with labeled HdU, preferably radioactive labeled; partially denaturing cellular DNA with a suitable denaturing agent; adding to the reaction medium mabs reactive with HdU; reacting the bound mabs with a labeled second antibody; incubating the mixture with a DNA stain; measuring simultaneously the intensity of the DNA stain as a measure of the total cellular DNA and the HdU incorporated as a measure of DNA synthesis. Measurements may be conveniently made by flow cytometry. Other techniques may also be utilized. The data obtained from the flow cytometric measurements are plotted to form bivariate distributions of DNA content vs. BrdU content. These distributions readily yield the relative amounts of the halodeoxyuridine (HdU) incorporated into cellular DNA during various phases of the cell cycle, namely, G 1 , S and G 2  M phases. The method also allows for the quantification of low levels of incorporation of HdU and is applicable for both in vivo and in vitro studies. Due to the sensitivity of the measurements, an analysis of both the DNA content and the level of incorporation of HdU in large cell populations as well as in single cells becomes possible. 
     The subject method is useful in many biological and biomedical investigations such as the detection of cells which have synthesized new DNA as part of their proliferation cycles; improved and accurate measurements of proliferation of cells in tumor and normal tissues, particularly monitoring the effects of drugs on proliferation in the course of human cancer therapy; the identification of cells that have repaired injury and the detection of unscheduled DNA synthesis; improved detection of chemicals which are potentially damaging to DNA; a more sensitive method of detecting chromosome injury to facilitate monitoring human cells for evidence of genetic injury; and for the assay of BrdU in solution, particularly in competition ELISA procedures to determine the level of BrdU in serum or other BrdU aqueous solutions as compared to the total cellular DNA content. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1a and 1b show bivariate DNA/BrdU distributions measured for CHO cells, before and after BrdU pulse. 
     FIG. 2 is a bivariate DNA/BrdU distribution diagram for CHO cells after a tritiated BrdU ([ 3  H]BrdU) treatment. 
     FIG. 3 illustrates bivariate DNA/BrdU distributions measured for CHO cells at intervals of two hours following [ 3  H]BrdU treatment. 
     FIG. 4 illustrates the relation between the amount of BrdU incorporated and the intensity of green fluorescence. 
     FIG. 5 shows cell cycle traverse rates for CHO cells. 
     FIG. 6 depicts the bivariate DNA/BrdU distributions measured for mouse bone marrow cells before and after treatment with araC. 
    
    
     DESCRIPTION OF THE INVENTION 
     The present invention is directed to a method for the simultaneous flow cytometric measurement of the cellular DNA content and the amount of the uptake of DNA precursors as a measure of DNA synthesis during various phases of the cell cycle and cell cycle traverse in normal and malignant cells in vitro and in vivo. A monoclonal antibody reactive with a DNA precursor, such as thymidine or an analog thereof, serves as a probe for newly synthesized DNA during various phases of the cell cycle such as G 1 , S and G 2  M. The DNA precursor normally utilized in these measurements is a halogenated uridine, preferably bromo-, chloro- or iododeoxyuridine, which is a chemical analog of thymidine. Halodeoxyuridine (HdU) is readily taken up by the cells and incorporated into their DNA in the place of thymidine. 
     A critical aspect of the invention is the partial denaturation of the cellular DNA to facilitate the accessibility of monoclonal antibodies to the incorporated HdU. While any suitable denaturants may be utilized, HCl, preferably, about 1.5M HCl is the denaturant of choice. The antibody does not bind to HdU in double stranded DNA. For the measurement of the total cellular DNA content, propidium iodide (PI) is used as the probe. Since PI also stains the RNA in the cell, it is necessary to remove the RNA content of the cell by acid or enzymatic hydrolysis. The HCl treatment not only partially denatures the DNA to facilitate antibody binding but also hydrolyzes RNA thereby eliminating separate RNAse or similar enzyme treatment. 
     The data obtained from the flow cytometric measurements are plotted to form bivariate distributions of DNA content vs. BrdU content. These distributions readily yield the relative amounts of the halodeoxyuridine incorporated into cellular DNA during various phases of the cell cycle, G 1 , S and G 2  M phases. The method also allows for the quantification of low levels of incorporation of HdU and is applicable for both in vivo and in vitro studies. The sensitivity of the measurements, makes possible an analysis of both the DNA content and the level of incorporation of HdU in large cell populations as well as in single cells. 
     The method of the subject invention for the simultaneous measurement broadly involves labeling cells, preferably pulse labeling, with radioactive labeled HdU, preferably tritiated HdU, still more preferably tritiated BrdU, or CldU or IdU and most preferably with tritiated BrdU, denaturing the DNA with any suitable denaturing agent, most conveniently HCl, preferably about 1.5M HCl, reacting the labeled cells with monoclonal antibodies reactive with HdU or more specifically, with BrdU or IdU, incubating the cells with a second antibody conjugated to a second label such as an enzyme or fluorescent dye, removing the excess of unbound, labeled second antibody, staining the cells with an appropriate DNA stain, preferably propidium iodide, which binds to the total DNA of the cell, measuring the level of the bound, labeled second antibody as a measure of the HdU or more specifically BrdU or IdU incorporated, and measuring the intensity of the bound DNA stain, preferably PrdI, as a measure of the total DNA content of the cell. The radioactivity of the bound tritiated HdU is measured by scintillation counting, preferably liquid scintillation counting, and compared and correlated with the level of HdU incorporation as measured by antibody binding. Monoclonal antibodies reactive with HdU are prepared according to the method described by M. Vanderlaan et al., in copending U.S. application Ser. No. 542,967, filed Oct. 18, 1983 which disclosure is incorporated herein by reference and made a part hereof. 
     The second antibody is usually goat antimouse antibody. The second label on the second antibody may be a fluorescent dye, a chromophore, a radionuclide or an enzyme. Exemplary fluorescent molecules are fluorescein and rhodamine and exemplary enzymes are alkaline phosphatase, peroxidase, preferably horse radish peroxidase and beta-galactosidase. When radioactive nuclides are used, the radioactivity is measured by liquid scintillation counting. When fluorescent molecules are used as the tags, the fluorescence intensities are measured by a fluorometer, a fluorescence cytometer or a spectrophotometer. When enzymes are used as the labels, measurements are made by standard ELISA techniques as described by M. Vanderlaan et al., supra. When peroxidase is the enzyme of choice, H 2  O 2  is used as a suitable substrate; when alkaline phosphatase is the enzyme used, the substrate of choice is NAD; and when beta-galactosidase is the enzyme, p-nitrophenyl-beta-D-galactopyranoside is the preferred substrate. 
     Fluorescein is the preferred label on the second antibody. The bound HdU, typically BrdU, is measured by measuring the fluorescence intensity of the fluorescein label at about 514 nm (low green) and the propidium iodide as a measure of the total DNA is measured in terms of its fluorescence intensity at about 600 nm (red). 
     The measurements are conveniently made using a flow cytometer and the data obtained are arranged to form bivariate distributions of DNA content vs. HdU content. These distributions readily yield the relative amounts of HdU incorporated during various phases of the cell cycle and the total DNA content of the cell. Analysis of the data yield information about cell cycle traverse and of the kinetics of DNA synthesis. The method of the subject invention is so sensitive that cell kinetics analysis may be obtained not only on large size cell populations but also on single cells. 
     The following examples are presented by way of example only and for purposes of illustration and are not to be construed as limiting the scope of the invention in any manner. 
     EXAMPLE 1 
     CHO Cell Culture: 
     Chinese hamster ovary (CHO) cells were grown in suspension culture in a one liter spinner flask in minimum essential alpha medium (alpha-MEM) with 10% fetal calf serum (FCS), to a density of 2×10 5  /ml. Bromodeoxyuridine (BrdU) was then added to the cell suspension to a final concentration of 10 uM. After 10 min the cells were spun down, washed once in phosphate buffered saline (PBS, pH 7.2) and resuspended in 100 ml PBS. The cells were then fractionated on the basis of size using a JE-6B elutriator rotor driven by a Beckman J2-2l centrifuge. Elutriation was carried out at room temperature with fractions collected on ice. Approximately 2×10 8  cells were loaded at a sedimentation velocity of 5.0 mm/hr×g, with a constant flow rate of 20 ml/min. Thereafter, the sedimentation velocity was increased in increments of 0.5 mm/hr×g with each collected fraction volume of 250 ml. Cells were then pelleted and resuspended in 70% EtOH. Elutriated fractions were stained for DNA using propidium iodide and incorporated BrdU was reacted with mouse monoclonal anti-BrdU-antibodies conjugated with fluoresceinated goat anti-mouse IgG as described in Examples 2 and 3 infra. 
     EXAMPLE 2 
     In Vitro Experiments with CHO Cells: 
     In one set of experiments, CHO cells growing exponentially in alpha-MEM with 10% FCS at 3×10 5  cells/ml were treated for 30 min with 10 uM BrdU. The cells were then refed with conditioned medium from a second untreated spinner flask. Twenty milliliter aliquots were removed at 2-hr intervals, counted, washed with phosphate buffered saline (PBS, pH 7.2), and fixed for at least 30 min in cold 70% ethanol (EtOH). 
     In a second series of experiments, exponentially growing CHO cells were treated with [ 3  H]BrdU for 30 min (specific activity, 35 Ci/mmol; 1 Ci=3.7×10 10  Bq; final concentration, 1 uCi/ml). The cells were then harvested, washed in PBS, and fixed for at least 30 min in cold 70% EtOH. 
     In a third series of experiments, the same procedure was repeated with mouse KHT tumor cells. 
     EXAMPLE 3 
     In Vivo Mice Experiments: 
     Control and 1-beta-D-arabinofuranosylcytosine (araC) treated female C3H mice were injected intraperitoneally with 50 mg of BrdU per kg one hour prior to sacrifice. araC-treated mice received BrdU 3, 6 and 9 hrs after a single intraperitoneal injection of 500 mg of araC per kg. The femurs of each mouse were removed and the hematopoietic cells were harvested by flushing with 1 ml of alpha-MEM. The recovered cells were then resuspended by pipeting, filtered through 37 um mesh, and fixed with 70% EtOH. 
     EXAMPLE 4 
     Monoclonal antibodies (mabs) to HdU, more specifically to BrdU, CldU or IdU were prepared according to the method of M. Vanderlaan et al., described in copending application U.S. Ser. No. 542,967, filed Oct. 18, 1983, which description is incorporated herein by reference and made a part hereof. The two hybridomas which continuously secrete the mabs reactive with HdU, more specifically with BrdU and IdU, are designated CIdU-1 and CIdU-2 and are on deposit with the American Type Culture Collection, 1230 Rocklawn Drive, Rockville, Md. and are accorded ATCC Accession Nos. HB-8321 and HB-8320 respectively. As used herein, however, the terms CIdU-1 and CIdU-2, refer not only to the hybridomas producing the mabs but also to the mabs reactive with HdU. 
     EXAMPLE 5 
     Cytochemistry: 
     The cells were removed from ethanol, resuspended in 2 ml of 1.5 M HCl at 20° C. for 20 min, washed twice with 5 ml of PBS, and resuspended for 1 hr in PBS solution containing 0.5% Tween 20, 0.5% bovine serum albumin (BSA) and a 1:200 dilution of monoclonal anti-BrdU. The cells were then washed twice with PBS and resuspended for 30 min in a solution containing PBS, 0.5% Tween 20, a 1:100 dilution of a stock solution of fluorescein-labeled goat anti-mouse gamma globulin and 1% neutral goat serum (to prevent nonspecific adsorption of the fluorescein-labeled anti-mouse gamma globulin). The cells were then washed twice with 5 ml of PBS and resuspended in 1 ml of PBS containing 5 ug of PrdI per ml. After 1 hr, the cells were ready for flow cytometric analysis. 
     EXAMPLE 6 
     Flow Cytometry and Cell Sorting: 
     Cell analysis was performed with a flow cytometer, designed and built at the Lawrence Livermore National Laboratory. Cell sorting was accomplished by using a fluorescence-activated cell sorter. During flow cytometry, cells were excited at 488 nm. Red fluorescence from PrdI was collected through a 600 nm long wavelength pass filter and recorded as a measure of total DNA content and green fluorescence from fluorescein was collected through a 514 nm bandpass filter and recorded as a measure of the amount of incorporated BrdU. The resulting data were accumulated to form a bivariate 64×64 channel distribution showing the distribution of total DNA (red fluorescence) and BrdU (green fluorescence) among the cells of the population. 
     Cells were sorted from several regions of the bivariate DNA/BrdU distribution curve measured for CHO cells treated for 30 min with [ 3  H]BrdU. Sorting regions are indicated in FIG. 3. Ten thousand cells were sorted from each region directly into liquid scintillation vials. The radioactivity per cell was determined in the manner described by J. W. Grey et al., J. Cell Physiol., 108, 135 (1981). 
     FIG. 1 shows the bivariate DNA/BrdU distributions measured for exponentially growing CHO cells before and after BrdU treatment. FIG. 1A is a diagram of the DNA/BrdU distribution prior to BrdU treatment and FIG. 1B shows the same DNA/BrdU distribution after a 30 min [ 3  H]BrdU pulse. The abscissae and the ordinates are univariate frequency distributions for the total DNA (red fluorescence) and the amount of incorporated BrdU (green fluorescence) respectively. The point of origin of each contour plot is indicated with an arrow. It can be seen that although all three phases of the cell cycle, the G 1 , S and G 2  phases show some BrdU incorporation (green fluorescence), there is a wide variation in BrdU incorporation, the S-phase cells in the labeled population showing a substantially larger level of BrdU incorporation (green fluorescence). The red fluorescence, indicating the total DNA content is about the same in all three phases. The bivariate distribution also indicates a variation in the rate of BrdU incorporation across S phase, the mid-S phase showing the highest level and the early and late S phase showing the lower levels. 
     FIG. 2 shows a plot of the green fluorescence intensity as a measure of BrdU incorporation vs. red fluorescence intensity as a measure of the total DNA content and the regions from which cells were sorted for liquid scintillation counting to measure the radioactivity per cell. The level of BrdU incorporated was measured by both mab binding and radioactivity of the bound [ 3  H]BrdU. 
     FIG. 3 illustrates the correlation between BrdU incorporated as measured by radioactivity of the tritiated BrdU as determined by liquid scintillation counting and green fluorescence as a measure of BrdU as measured by mab binding. A correlation of about 97% (r=0.97) confirms that a measure of the green fluorescence is an accurate measure of BrdU incorporation. 
     Cell cycle traverse rates can be calculated from the bivariate distribution curves of DNA/BrdU measured at specific time intervals following pulse labeling of the cells as illustrated in FIG. 4. The diagram (FIG. 4A through 4F) shows bivariate DNA/BrdU distributions measured at two hour intervals for exponentially growing CHO cells after a 30 min exposure to 1 uM [ 3  H]BrdU. The data indicate that the labeled CHO cells move out of the S phase into the G 2  M phase immediately or shortly after pulse labeling. About 8 hours after labeling, they are predominantly in the G 2  M and G 1  phases. By about 10 hours they start reentering the S phase. As the cells go through mitosis, the green fluorescence (BrdU content) is also seen to become divided between the divided cells. The BrdU content per cell is highest in mid-S phase and increases or decreases as the cohort of cells passes in or out of the G 1  phase. Cell cycle phase durations are quantitatively estimated from changes in the average values of BrdU per cell in mid-S phase and of BrdU per cell in the G 1  phase. These values may also be verified by measuring the amount of BrdU incorporated as a measure of the radioactivity per cell. 
     FIG. 5A through 5F show bivariate histograms of total DNA content (propidium iodide stain) vs. BrdU content (fluoresceinated antibody binding) for a control population of asynchronously growing CHO cells. The G 1  and G 2  M phases can be seen to be clearly separated from the S phase cells. The presence of G 1  and G 2  M peaks indicate a minor amount of nonspecific binding of the second antibody. 
     FIG. 6 shows isometric plots of the bivariate DNA/BrdU distributions measured for bone marrow cells obtained from control C3H mice and from araC-treated C3H mice labeled with BrdU 3, 6 and 9 hours after araC treatment. The total DNA content is plotted on the x axis, BrdU on the y axis and frequency on the z axis. All distributions show two distinct peaks corresponding to G 1  DNA content--one at low green fluorescence and one at high green fluorescence. Cells sorted from the low green fluorescence peak were determined by microscopic analysis to be lymphocytes and blast cells, whereas cells sorted from the high green fluorescence peak were determined to be mostly well-differentiated granulocytes and macrophages. In the control population, cells sorted from mid-S phase were determined to be predominantly blast cells and thus to be associated with the low green fluorescence peak and with G 1  -phase DNA content. The DNA/BrdU distribution measured for cells treated with BrdU 3 hours after araC treatment shows that the majority of cells with an S-phase DNA content are characterized by low green fluorescence values, indicating negligible levels of DNA synthesis (i.e., BrdU incorporation). However, the DNA/BrdU distribution measured for cells treated with BrdU 6 hr after araC treatment shows that most of the cells with S-phase DNA content that did not incorporate BrdU have disappeared. The appearance in early S phase of a population incorporating substantial amounts of BrdU can also be seen in this distribution. The DNA/BrdU distribution measured for cells treated with BrdU 9 hr after araC treatment shows a substantial population of cells in early S phase that have incorporated a greater than normal amount of BrdU, suggesting an elevated rate of DNA synthesis. 
     The subject invention thus provides a sensitive method for the simultaneous measurement of the total cellular DNA content and of the level of DNA synthesis. The method is useful in many biological and biomedical investigations such as the detection of cells which have synthesized new DNA as part of their proliferation cycles; improved and accurate measurements of proliferation of cells in tumor and normal tissues, particularly monitoring the effects of drugs on proliferation in the course of human cancer therapy; the identification of cells that have repaired injury and the detection of unscheduled DNA synthesis; improved detection of chemicals which are potentially damaging to DNA; a more sensitive method of detecting chromosome injury to facilitate monitoring human cells for evidence of genetic injury; and for the assay of BrdU in solution, particularly in competition ELISA procedures to determine the level of BrdU in serum and other biological fluids or other BrdU aqueous solutions as compared to the total cellular DNA content. 
     The foregoing description of the preferred embodiments of the subject invention have been presented for purposes of illustration and description and for a better understanding of the invention. It is not intended to be exhaustive or to limit the invention to the precise form disclosed; and obviously, many modifications and variations are possible in light of the above teaching. The particular embodiments were chosen and described in some detail to best explain the principles of the invention and its practical application to thereby enable others skilled in the relevant art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the invention be defined by the claims appended hereto.