TP53 and breast cancer: In two other transgenic mouse models of breast cancer, where tumors were initiated by activated Harvey rat sarcoma viral oncogene homolog (v-Ha-ras) (Hundley et al. 1997) or wingless-related murine mammary tumor virus (MMTV) integration site 1 (Wnt-1) (Donehower et al. 1995), inactivation of p53 did not result in a reduction of apoptosis; rather, loss of p53 was associated with increased proliferation of the mammary epithelium.