In this study we show that mammary tumors caused by inactivation of the pRb family (pRbf) of proteins (pRb, p107, p130), together with Brca1 and p53 inactivation, mimic several aspects of the most aggressive forms of breast cancer, including rapid tumor progression, poor differentiation, distant metastasis, necrosis, metaplasia, and genomic instability. This evidence concerns the gene RBL1 and breast cancer.