the present invention is directed to a mechanism for marking biological samples that are to be used for subsequent nucleic acid analysis . the method involves adding a nucleic acid molecule of known sequence to the biological sample at the time of sample collection . the method further utilizes primers specific to the complementary strands of the added dna , such that they will direct the synthesis of another dna molecule of known length when used in a standard or multiplex polymerase chain reaction . this provides an unambiguous identifying label for the collected forensic or medical samples , including blood , semen , saliva , urine , tissue , and mixtures of bodily fluids . when used with the supplied primers or dna probe , pcr or nucleic acid hybridization techniques will produce or recognize dna fragments of predetermined size , preventing errant confusion of said samples with other forensic or medical samples that do not contain the aforementioned dna additive .