The efficacy of the three CDK4/6i, palbociclib, ribociclib, and abemaciclib, was tested in breast cancer cell lines that represent an AI-resistant/long-term estrogen deprived (LTED) setting and harbor either wild-type (HCC1428-LTED, abbreviated as ESR1wt) or mutant (MCF7-LTED-D538G and MCF7-LTED-Y537S, abbreviated as ESR1mut: D538G and ESR1mut: Y537S, respectively) ER. Here, CDK4 is linked to breast cancer.