This domain family is found in bacteria and eukaryotes, and is approximately 180 amino acids in length. This domain is the WW domain of PCIF1. PCIF1 interacts with phosphorylated RNA polymerase II carboxy-terminal domain (CTD). 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.