TP53 and neoplasm: Although loss of p53-dependent apoptosis accounts for the acceleration of mammary tumorigenesis in WAP-T121;p53+/− mice, in models expressing either activated v-Ha-ras (Hundley et al. 1997) or Wnt-1 (Jones et al. 1997), earlier tumor formation in p53 heterozygous and homozygous null mice is accounted for by increased proliferation rather than attenuated apoptosis.