Phospho-Rpb1 (Thr4) Antibody recognizes endogenous levels of Rpb1 protein only when the carboxy-terminal domain (CTD) heptapeptide repeat [Tyr1, Ser2, Pro3, Thr4, Ser5, Pro6, Ser7] is phosphorylated at Thr4. This antibody does not cross-react with Rpb1 CTD phosphorylated at Ser2, Ser5 or Ser7.Polyclonal antibody is produced by immunizing animals with a synthetic phosphopeptide corresponding to residues surrounding Thr4 of human Rpb1 CTD heptapeptide repeat. Antibodies are purified by protein A and peptide affinity chromatography.