Inhibitor of PPP1CA. Has inhibitory activity only when phosphorylated, creating a molecular switch for regulating the phosphorylation status of PPP1CA substrates and smooth muscle contraction (By similarity). Isoform 1 is detected in intestine, kidney, lung, spleen, stomach and brain. Isoform 2 is detected in testis. Phosphorylated on several residues. Belongs to the PP1 inhibitor family.