CDK9 and breast carcinoma: In endocrine-resistant ERα+ breast cancer, CDK9 mediates hyperphosphorylation of serine-2 in the RNA Pol II CTD at the c-MYC gene promoter, resulting in transcriptional elongation and overexpression of c-MYC, which drives estrogen-independent growth of resistant cancer cells (Sengupta et al., 2014).