CDK9 and cancer: Flavopiridol is an inhibitor of cyclin-dependent kinases (CDKs) (including CDK1, CDK2, CDK4, CDK5, CDK6, CDK7, CDK8, and CDK9) and has been investigated for the treatment of several types of cancers, including leukemia, liver cancer, and renal cancer, in clinical phase 2 trials (24, –, 26).