CDK9 and cancer: However, depletion of CDK9 in cancer cell lines, lead to cell cycle delay with an accumulation in G1 and corresponding decrease in S phase.145 Moreover, knockdown of CDK9 in Drosophila cells induces cell cycle arrest in G1.183 Additionally, CDK9 phosphorylates the RB protein in vitro and in vivo, which can influence Pol II dependent gene transcription.184 Interestingly, independently from canonical interaction with cyclin T, CDK9-cyclin K activity seems to be necessary for cell cycle recovery after replication stress.