Our main observations in breast cancer cell lines were that palbociclib and ribociclib had identical dose-dependent proliferation inhibition and that in CDK4/6 inhibitor-sensitive cell lines, both inhibitors reduced the levels of p-RB1 (marker of cell cycle inhibition21) and Lamin-B1 (marker of senescence40–42) in a similar manner, where treatment duration played a part but changes mostly relied on the administered doses. The gene discussed is CDK4; the disease is breast carcinoma.