In vitro, in the OC cell lines OVCAR-429 and PA-1, melatonin treatment (0–800 μM) resulted in an accumulation of cells in the G1 phase of the cell cycle in parallel with a downregulation of CDK2 and CDK4, another finding that contributes to explain the antitumor activity of melatonin in ovarian cancer [115]. The gene discussed is CDK4; the disease is ovarian cancer.