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). Here, CDK9 is linked to breast carcinoma.