Taken together, the cell cycle arrest in the two human melanoma cell lines SK-MEL-28 and WM115 is either mediated indirectly by the induction of p21, which inhibits various CDKs, as in the case of cytokines and doxorubicin, or directly by the synthetic CDK4/6 inhibitor palbociclib. The gene discussed is CDK4; the disease is melanoma.