CDK-IN-18 (Compound EX.161) is an orally active, brain-penetrant, and selective inhibitor of CDK2, CDK4, and CDK6. It hinders the activity of cyclin-CDK complexes, preventing the phosphorylation of retinoblastoma protein (pRb), and thereby blocks the transition of cells from the G1 phase to the S phase. CDK-IN-18 shows potential for cancer research, such as in breast cancer (particularly hormone receptor-positive, HER2-negative types), lung cancer, and colorectal cancer.