The functionality of the mammary-specific activation of the EF1-tTA was tested in two biological experiments: (1) the role of active STAT5 as a survival factor for secretory alveolar cells and its ability to delay the postlactational remodeling of the gland, and (2) the induction of multifocal mammary tumors in response to the expression of mutant KRAS. Here, KRAS is linked to breast cancer.