Flavopiridol inhibits the phosphorylation of RNA polymerase II CTD. CLL lymphocytes were incubated with 3 μM flavopiridol for 24 or 48 hours. The phosphorylation of RNA polymerase II was analyzed by immunoblotting, using antibodies towards the phosphorylated Ser2 or Ser5 sites of the CTD, as well as total RNA polymerase II. (A) Representative blot from patient no. 7. (B) Action of flavopiridol on the phosphorylation status of RNA polymerase II. The immunoblots were quantified by densitometry. Levels of phosphorylation were normalized to the loading control β-actin, and then expressed as percentage of controls incubated with DMSO. Five CLL samples were incubated with 3 μM flavopiridol for 48 hours as well, and 4 samples incubated with flavopiridol for 24 hours only are shown. • indicates 24 hours; ▴, 48 hours.