We showed that following Rb perturbation in vivo, tumor progression is limited largely by p53-dependent apoptosis, and that loss of the second p53 allele in MFT121+/p53f/+ tumors is likely a prerequisite for mammary tumor progression [19], since the vast majority of tumors lose the wild type p53 allele during tumorigenesis. Here, RB1 is linked to neoplasm.