Calcineurin, a phosphatase composed of two subunits, including the catalytic subunit A and the regulatory subunit B, is involved in calcium signal transduction pathways. It is located in the cytoplasm and nucleus and functions by dephosphorylating key proteins such as NFAT (nuclear factor of activated T cells), regulating cell proliferation, differentiation, and apoptosis. Mutations or dysfunction in calcineurin are associated with various diseases, including immune system disorders, cardiovascular diseases, and neurodegenerative diseases.