Regulatory subunit of calcineurin, a calcium-dependent, calmodulin stimulated protein phosphatase. Confers calcium sensitivity. Forms a complex composed of a calmodulin-dependent catalytic subunit (also known as calcineurin A) and a regulatory Ca(2+)-binding subunit (also known as calcineurin B). There are three catalytic subunits, each encoded by a separate gene (PPP3CA, PPP3CB, and PPP3CC) and two regulatory subunits which are also encoded by separate genes (PPP3R1 and PPP3R2) (By similarity). Interacts with SPATA33 (via PQIIIT motif) (PubMed:34446558). Localizes in the mitochondria in a SPATA33-dependent manner. Testis-specific. This protein has four functional calcium-binding sites. Belongs to the calcineurin regulatory subunit family. Extended N-terminus. Extended N-terminus. Extended N-terminus. Extended N-terminus. Extended N-terminus.