ESR1 and breast cancer: In estrogen receptor-positive (ER+) breast cancer cells, it has been shown that estrogen-ER recruits the positive transcriptional elongation factor-b (pTEFb) to phosphorylate, via its catalytic component CDK9, serine 2 of the C-terminal-domain (CTD) of Pol II to overcome the elongation block17 and allow MYB expression.