Calcitriol reduced cell proliferation in cancer cells by increasing the CDK inhibitors p21 and p27, leading to lower CDK2 activity, the hypophosphorylation of the retinoblastoma protein (RB1) [98,99], and the downregulation of several cyclins: CCND1-encoding cyclin D1, CCND3-encoding cyclin D3, CCNA1-encoding cyclin A1, and CCNE1-encoding cyclin E1. The gene discussed is RB1; the disease is cancer.