A cell-permeable triaminopyrimidine that selectively and reversibly blocks Cdk4/cyclin D1 and Cdk6/cyclin D1 activity (IC50s = 1.5 and 5.6 μM, respectively); induces cell cycle arrest in the G1 phase and apoptosis in asynchronous cell lines; suppresses tumor growth in mice bearing human HCT116 colon carcinoma xenografts.