Phosphorylation of the carboxy-terminal domain (CTD) of RNA polymerase II (RNAP II) largest subunit has an important role. in transcription elongation and in coupling transcription to pre-mRNA processing. To identify proteins that can directly bind to the phosphorylated CTD, we screened a human cDNA expression library using P-32-labeled CTD as a probe. Here we report the cloning and characterization of a novel human WW domain-containing protein, PCIF1 (phosphorylated CTD interacting factor 1). PCIF1 is composed of 704 amino acids. The WW domain of PCIF1 can directly and preferentially bind to the phosphorylated CTD compared to the unphosphorylated CTD. PCIF1 binds to the hyperphosphorylated RNAP II (RNAP IIO) in vitro and in vivo. Double immunofluorescence labeling in HeLa cells demonstrated that PCIF1 and endogenous RNAP IIO are co-localized in the cell nucleus. Thus, PCIF1 may play a role in mRNA synthesis by modulating RNAP IIO activity. (C) 2003 Elsevier Science (USA). All rights reserved.