CTDSP1 also named as NIF3, NLIIF, or SCP1, is a 261 amino acid protein, which contains one FCP1 homology domain. CTDSP1 localizes in the nucleus and colocalizes with RNA polymerase II. CTDSP1 is expressed in non-neuronal tissues. It is highly expressed in skeletal muscle, spleen, lung and placenta. CTDSP1 preferentially catalyzes the dephosphorylation of 'Ser-5' within the tandem 7 residues repeats in the C-terminal domain (CTD) of the largest RNA polymerase II subunit POLR2.