Regulatory subunit of protein phosphatase 1. Inactivates the PPP1CC isoform 2 during epididymal sperm maturation. Interacts with PPP1CA, PPP1CB and PPP1CC/PPP1G (By similarity). Interacts with PPP1CC isoform 2 in motile caudal epididymal spermatozoa. Expressed in epididymal spermatozoa including the principal piece of the flagellum and the head-neck junction. Belongs to the SDS22 family.