CDK4 and cancer: However, in tumors without proficient RB CDK4/6 inhibition is not effective.26 Furthermore, TP53 mutations appeared to be strongly related to CDK4/6 inhibitor resistance in a panel of 560 cancer cell lines treated with palbociclib and abemaciclib.27 This is in line with our findings and could explain the worse PFS observed in the TP53 mutated cases.