CDK9 and cancer: Through kinase selectivity, physicochemical properties, DMPK properties, in vitro and in vivo efficacy, 55 is the first potent and highly selective PTEFb/CDK9 (IC50 CDK9/CycT1: 13 nm, ratio of IC50 values CDK2/CDK9: 100) inhibitor to enter clinical trials for the treatment of cancer.