Regulatory sequences derived from the rat whey acidic protein gene have been used to preferentially overexpress a murine 172Arg-->Leu mutant p53 in the mammary gland of transgenic mice. Several different lines of mice expressing the 172Arg-->Leu mutant p53 displayed an impaired ability to lactate, and the mice expressing the highest levels of mutant p53 were unable to nurse their young. This failure was related to the inhibition of normal lobuloalveolar development that occurred during late pregnancy and a marked decrease in milk protein gene expression at early lactation. Interestingly, immunohistochemical analysis revealed that the mutant p53 was localized predominantly in the cytoplasm of alveolar cells. Ductal development was not overtly impaired in these mice. Expression of the 172Arg-->Leu mutant p53 resulted in radiation-induced apoptosis, and transactivation or repression of the expression of a number of genes, including mdm-2 and proliferating cell nuclear antigen, known properties of wild-type p53. The availability of lines of mice preferentially expressing specific p53 mutants in the mammary gland should facilitate evaluation of the roles of other factors, such as hormones, oncogenes and chemical carcinogens, in the etiology of breast cancer.