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) (PubMed:26794871). 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). The regulatory subunit confers calcium sensitivity. Interacts with catalytic subunit PPP3CA/calcineurin A (PubMed:26794871). Interacts with catalytic subunit PPP3CB/calcineurin A (By similarity). Isoform 1 and isoform 2 interact with CIB1 (via C-terminal region); the interaction increases upon cardiomyocyte hypertrophy (PubMed:20639889). Interacts with SPATA33 (via PQIIIT motif) (By similarity). Translocates from the cytosol to the sarcolemma in a CIB1-dependent manner during cardiomyocyte hypertrophy. Expressed in osteoblasts and bone marrow (at protein level) (PubMed:16286645). Expressed in the brain, kidney, liver, lung, muscle, ovary, spleen, thymus, heart and testis (PubMed:1325794). This protein has four functional calcium-binding sites. Belongs to the calcineurin regulatory subunit family.