We recently established transgenic animals (WAP-SV-T/t) carrying the early coding region of Simian Virus 40 (SV40) under the transcriptional control of the whey acidic milk protein promoter (WAP), which restricts the expression of the transgene to mammary gland epithelial cells (ME-cells). SV40 T/t-antigen synthesis causes premature mammary gland involution during late pregnancy by inducing apoptosis and leads to development of mammary tumors after the first lactation period in both p53 positive (WAP-SV-T/t) and p53 negative double transgenic animals (WAP-SV-T/t.p53-/-). The high apoptotic rate persists in all of the T/t-antigen positive breast tumor cells, as well as in established ME-tissue culture cell lines. ME-cells which spontaneously switch off the expression of the WAP-SV-T/t transgene do not undergo apoptosis. However, these cells again exhibit an extensive DNA fragmentation when SV40 T/t-antigen synthesis is reintroduced, which indicates that it is the expression of T/t antigen which is the critical factor for induction of apoptosis. In addition, we isolated several ME-cell lines from different breast tumors which have spontaneously lost the T/t-antigen yet remain maximally transformed. Strikingly, these cells contain a missense mutation of the p53 gene at codon 242 (p53(242)), which substitutes alanine for glycine. This mutation increases p53 stability and it reduces the transactivating function of p53, albeit without affecting the ability of the protein to interact with the DNA. This indicates that p53 missense mutations are selected for in breast tumors initially expressing T/t-antigen. Therefore, the p53(242) mutation is sufficient to maintain the transformed state after the ME-cells have switched off the WAP-SV-T/t transgene. Interestingly, the p53 minus state per se is not sufficient to induce ME-cell transformation since homozygous null mice for the p53 gene (p53-/-) fail to develop breast cancer.