PD-0332991 is designed to inhibit the cyclin D-cyclin-dependent kinases 4 and 6 (CDK4/6)-retinoblastoma (RB) pathway [23]; RB1 expression is an established clinical predictor of sensitivity to this drug, which remains under rigorous clinical investigation in RB expressing breast cancers, where some have reported a doubling of progression-free survival time [24, 25]. This evidence concerns the gene CDK4 and breast cancer.