CTDSP1 is a class 2C phosphatase with activity dependent on the conserved DxD motif. 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 POLR2A. In addition, CTDSP1 negatively regulates RNA polymerase II transcription, possibly by controlling the transition from initiation/capping to processive transcript elongation.