CDK9 and breast cancer: The inhibitory effect on MYB gene activity by these inhibitors is most likely a result of blocked transcription, since MYB gene expression is known to be positively regulated by the transcriptional elongation factor P-TEFb (which includes CDK9) and is inhibited by CDKi (e.g. AT7519 and flavopiridol, also identified in our screen) in breast cancer cells (37, 38).