CDK1 and cancer: THZ1 is a selective inhibitor of CDK7 that has demonstrable anti-tumour activities for a wide range of cancer types,43 acting to inhibit transcription by preventing phosphorylation of the RNA polymerase II (PolII) C-terminal domain heptapeptide repeats, and affecting cell cycle progression by inhibiting CDK7-mediated phosphorylation of CDK1, CDK2, CDK4 and CDK6.44 THZ1 inhibited MCF7 cell growth (GI50=188 nM) (Figure 5b).