the present invention provides a method for treating mammary gland tumors in a subject by stimulating an immune response to a mammary gland - associated differentiation antigen . the subject is preferably canine or feline , although the invention can be applied to other animal species , preferably mammalian or avian species , as well . as used in the specification and claims of this application , the term “ immune response ” encompasses both cellular and humoral immune responses . preferably , the immune response is sufficient to provide immunoprotection against growth of tumors expressing the target differentiation antigen . the term “ stimulate ” refers to the initial stimulation of a new immune response or to the enhancement of a pre - existing immune response . in accordance with the invention , a subject is treated by administering a xenogeneic differentiation antigen of the same type as a target differentiation antigen expressed by mammary gland tumor cells of the subject in an amount effective to stimulate an immune response . thus , for example , if the target differentiation antigen is the her2 / neu antigen found in mammary cells , the therapeutic antigen is a xenogeneic her2 / neu antigen . in one embodiment , the inventive method may include the following steps : ( 1 ) immunization to an animal in need of a xenogeneic antigen , for example , the rat her2 / neu as set forth in seq id no : 2 and encoded by nucleotides 106 - 3885 of the sequence as set forth in seq id no : 1 , ( 2 ) needle - free priming of immune responses , ( 3 ) electrotransfer - based booster , and ( 4 ) vaccination after tumor debulking by surgical primary therapy . in another embodiment , the inventive method is carried out on subjects , including companion animals , without metastasis ( i . e . in relatively early stages of mammary carcinoma disease progression ). in some embodiments , the boost comprises administering plasmids encoding xenogeneic antigens , for example those encoding rat her2 protein ( seq id no : 2 ). in some embodiments , the xenogeneic antigen is encoded by a nucleotide having favorable nucleotide substitutions with respect to the sequence as set forth in seq id no : 1 . favorable substitutions include any changes that result in improved immune response against the her2 / neu expressed by the cells of the mammary tumor / carcinoma . substitutions can include existing sequences , such as murine her2 ( seq id no : 3 ), human her2 ( seq id no : 4 ), or any other xenogeneic her2 sequence , or fragment thereof , capable of eliciting a therapeutically effective immune response in a target animal against a her2 - associated mammary carcinoma . in some embodiments , the boost comprises administering a xenogeneic differentiation antigen . in other embodiments , the boost comprises administering a syngeneic differentiation antigen . xenogeneic differentiation antigen may be administered as a purified differentiation antigen derived from the source organism . proteins can be purified for this purpose from cell lysates using column chromatography procedures . proteins for this purpose may also be purified from recombinant sources , such as bacterial or yeast clones or mammalian or insect cell lines expressing the desired product . administration of the xenogeneic differentiation antigen can be accomplished by several routes . first , the xenogeneic differentiation antigen may be administered as part of a vaccine composition which may include one or more adjuvants such as alum , qs21 , titermax or its derivatives , incomplete or complete freund &# 39 ; s and related adjuvants , and cytokines such as granulocyte - macrophage colony stimulating factor , flt - 3 ligand , interleukin - 2 , interleukin - 4 and interleukin - 12 for increasing the intensity of the immune response . the vaccine composition may be in the form of a xenogeneic differentiation antigen in a solution or a suspension , or the therapeutic differentiation antigen may be introduced in a lipid carrier such as a liposome . such compositions will generally be administered by subcutaneous , intradermal or intramuscular route . vaccine compositions containing expressed xenogeneic differentiation antigen are administered in amounts which are effective to stimulate an immune response to the target differentiation antigen in the subject . the preferred amount to be administered will depend on the species of the subject and on the specific antigen , but can be determined through routine preliminary tests in which increasing doses are given and the extent of antibody formation or t cell response is measured by elisa or similar tests . t cell responses may also be measured by cellular immune assays , such as cytotoxicity , cytokine release assays and proliferation assays . the xenogeneic differentiation antigen may also be introduced in accordance with the invention using a dna immunization technique in which dna encoding the antigen is introduced into the subject such that the xenogeneic differentiation antigen is expressed by the subject . cdna encoding the differentiation antigen is combined with a promoter which is effective for expression of the nucleic acid polymer in mammalian cells . this can be accomplished by digesting the nucleic acid polymer with a restriction endonuclease and cloning into a plasmid containing a promoter such as the sv40 promoter , the cytomegalovirus ( cmv ) promoter or the rous sarcoma virus ( rsv ) promoter . the resulting construct is then used as a vaccine for genetic immunization . the nucleic acid polymer could also be cloned into plasmid and viral vectors that are known to transduce mammalian cells . these vectors include retroviral vectors , adenovirus vectors , vaccinia virus vectors , pox virus vectors and adenovirus - associated vectors . the nucleic acid constructs containing the promoter and the antigen - coding region can be administered directly or they can be packaged in liposomes or coated onto colloidal gold particles prior to administration . techniques for packaging dna vaccines into liposomes are known in the art , for example from murray , ed . “ gene transfer and expression protocols ” humana pres , clifton , n . j . ( 1991 ). similarly , techniques for coating naked dna onto gold particles are taught in yang , “ gene transfer into mammalian somatic cells in vivo ”, crit . rev . biotech . 12 : 335 - 356 ( 1992 ), and techniques for expression of proteins using viral vectors are found in adolph , k . ed . “ viral genome methods ” crc press , florida ( 1996 ). for genetic immunization , the vaccine compositions are preferably administered intradermally , subcutaneously or intramuscularly by injection or by gas driven particle bombardment , and are delivered in an amount effective to stimulate an immune response in the host organism . the compositions may also be administered ex vivo to blood or bone marrow - derived cells ( which include apcs ) using liposomal transfection , particle bombardment or viral infection ( including co - cultivation techniques ). the treated cells are then reintroduced back into the subject to be immunized . while it will be understood that the amount of material needed will depend on the immunogenicity of each individual construct and cannot be predicted a priori , the process of determining the appropriate dosage for any given construct is straightforward . specifically , a series of dosages of increasing size , starting at about 0 . 1 μg is administered and the resulting immune response is observed , for example by measuring antibody titer using an elisa assay , detecting ctl response using a chromium release assay or detecting th ( helper t cell ) response using a cytokine release assay . once tolerance is broken through the administration of the xenogeneic differentiation antigen , subsequent treatments with syngeneic differentiation may be employed to maintain and in some cases enhance the immune response . ( see , weber , et al ., “ tumor immunity and autoimmunity induced by immunization with homologous dna .” j clin invest 102 ( 6 ): 1258 ( 1998 ).) thus , in one embodiment of the invention , the subject is first treated by administration of a xenogeneic differentiation antigen ( for example for three treatment cycles ), and subsequently by administration of a syngeneic differentiation antigen ( for example for an additional three treatment cycles ). as an alternative to treatment cycles using different therapeutic agents , one can use a single therapeutic agent containing both xenogeneic and syngeneic differentiation antigens . thus , for example , a mixture of the rher2 - ping and hher2 - ping vectors , or a single vector encoding both rat and human her2 / neu under the control of a promoter such that they are expressed in a canine subject can be employed for the treatment of mammary gland tumor in canines . vectors are available commercially , for example from stratagene and other companies , which can express two independent genes . commonly , these vectors use an internal ribosomal entry site , or ires , between the two genes . this approach has the advantage of requiring approval for only a single therapeutic agent . all documents cited herein are herein incorporated by reference in their entirety . the invention will now be further described with reference to the following , non - limiting examples . the extracellular domain of rat her2 / neu ( nucleotides 17 - 3799 of seq id no : 1 ) was amplified by pcr from the pcmvneunt ( amici et al ., 1998 ) plasmid using the primers forward : 5 ′- cgaagcttaccatggagctggcggcctgg - 3 ′ ( seq id no : 6 ) and reverse : 5 ′- cggaattcttatgtcaccgggctggc - 3 ′ ( seq id no : 7 ). the hindiii - ecori fragment was cloned into pcdna3 . 1 (+) ( invitrogen , carlsbad , calif . ; and fig2 ). the original sequence of the rat neu cdna was described previously ( bargmann et al ., 1986 ), and is herein set forth in seq id no : 1 , with the coding sequence from nucleotides 17 to 3799 . the rat her2 / neu coding sequence was then subcloned into the ping vector ( bergman et al ., clin cancer res , 9 : 1284 - 1290 , 2003 , backbone depicted in fig3 ; map depicted in fig3 a ; and sequence as set forth in seq id no : 5 ), to yield rat her2 / neu - ping . in this trial , 10 dogs with mgt were enrolled and immunized with 100 μg of ping - rher2 dna per dose . the signalment for these dogs is set forth in table 1 and the tumor staging is set forth in table 2 . table 2 tumor staging tumor size ( cm ) mgt type stage mgt 01 2 × 2 × 4 tubulopapillary carcinoma t 3 n 0 m 0 0 . 2 × 0 . 2 × 0 . 2 0 . 2 × 0 . 3 × 0 . 2 0 . 1 × 0 . 1 × 0 . 1 0 . 5 × 0 . 5 × 0 . 5 0 . 2 × 0 . 2 × 0 . 2 0 . 5 × 0 . 5 × 0 . 5 mgt 02 12 × 10 × 8 lipid rich carcinoma t 3 n 0 m 0 5 × 3 × 1 . 5 1 × 1 × 1 1 × 1 × 0 . 5 0 . 5 × 0 . 1 × 0 . 1 mgt 03 5 . 6 × 4 . 8 × 4 . 6 tubulopapillary carcinoma with t 3 n 0 m 0 1 . 8 × 1 . 5 × 1 . 2 fibroadenoma mgt 04 4 . 2 × 5 . 6 × 2 . 5 tubulopapillary carcinoma t 3 n 0 m 0 mgt 05 1 . 2 × 1 × 0 . 5 simple adenoma t 1 n 0 m 0 1 × 1 . 4 × 0 . 5 1 × 1 × 0 . 4 0 . 5 × 0 . 5 × 0 . 5 mgt 06 10 × 4 × 3 lipid rich carcinoma with t 3 n 0 m 0 fibroadenoma mgt 07 1 × 1 × 1 complex type t 1 n 0 m 0 0 . 5 × 0 . 5 × 0 . 5 mgt08 1 × 1 × 1 complex type t 1 n 0 m 0 0 . 5 × 0 . 5 × 0 . 5 mgt09 2 . 5 × 2 × 1 complex type t 1 n 0 m 0 1 . 5 × 2 × 1 mgt10 1 × 1 × 1 tubulopapillary carcinoma t 1 n 0 m 0 0 . 5 × 0 . 5 × 0 . 5 0 . 1 × 0 . 1 × 0 . 1 as indicated , this group included five stage i and five stage iii dogs , which all received three doses of vaccine at two week intervals . the first and second doses were administered with the vitajet ™ transdermal device and the third dose by intramuscular injection concurrent with electroporation . vaccination was initiated following surgical removal of the mgt with concurrent ovariohysterectomy ( ohe ). all dogs were negative for regional lymph node and pulmonary metastasis . disease free survival and overall survival times were calculated using day of surgery as day 0 with results presented in table 3 . a group of 19 dogs was identified as historical control cases . all control dogs underwent surgical removal of mgt with concurrent ohe and were negative for regional lymph node and pulmonary metastasis . this group included 7 stage i , 3 stage ii , and 9 stage iii dogs . disease free and overall survival times were calculated for these dogs using day of surgery as day 0 . the signalment for these dogs is set forth in table 4 and tumor staging for each dog is set forth in table 5 . disease free and overall survival times were calculated for the control group and are presented in fig1 a - 1c . philibert et al . ( 2003 ) reviewed survival statistics for 97 dogs with mgt and reported median survival times for 41 dogs with mgt less than 3 cm in diameter to be 22 months (˜ 666 days ) versus 14 months (˜ 424 days ) for 56 dogs with mgt greater than 3 cm in diameter . in the absence of lymph node involvement or metastasis , tumor size less than 3 cm correlates with stage i disease and greater than 3 cm correlates with stage ii or higher disease status . they did not find a difference in survival time for dogs in stages ii , iii or iv . overall median survival time for all dogs treated with the ping - rher2 vaccine is 678 days . this was significantly higher as compared to the historical data from the 19 dogs provided by ntu indicating a median overall survival time of 300 days , and to the data published by philibert et al . ( 2003 ) indicating 424 days overall survival time for dogs with stage ii or greater mgt . the ping - rher2 dna vaccine will target dogs and cats with tumors shown to over express the her2 antigen based upon tumor tissue analysis using existing her2 tissue expression assays . the vaccine will be administered using the vetjet ™ transdermal device to deliver 100 μg of dna into the medial thigh of dogs or lateral thigh of cats , at two week intervals for four doses . dogs and cats that survive will receive a booster dose every six months . the invention will now be described by the following non - limiting claims .