Inhibitor of PPP1CA. Has over 50-fold higher inhibitory activity when phosphorylated. Ubiquitous. Highly expressed in testis. Detected at low levels in the other tissues tested. Highly expressed in cardiac muscle, bladder and aorta (at protein level). Phosphorylated primarily on Thr-57 by PKC (in vitro). An unknown Ser is also phosphorylated by PKC (in vitro). Belongs to the PP1 inhibitor family.