RB1 and breast cancer: Interestingly, though loss-of-function of the p18-cyclin D/CDK4-Rb pathway is a common event in both luminal and basal-like breast cancers, loss of or mutation of Rb per se is mainly detected in BLBCs, which also explains why clinically defined luminal type tumors, but not basal-like tumors, are more sensitive to CDK4 inhibitors since CDK4 promotes cell proliferation dependent on functional RB.