A somewhat reductionist model of Rb’s role in tumour suppression suggests that loss of Rb’s E2F repressive function allows premature transition of cells through the G1 cell cycle checkpoint; it also seems likely that loss of Rb function in breast cancer also influences additional processes that contribute to the development of the disease, including the differentiation of stem and progenitor cells and the transition of cells from an epithelial to a mesenchymal phenotype [3]. This evidence concerns the gene RB1 and breast cancer.