A phosphatase enzyme that plays a crucial role in relaying intracellular signals following activation of immune cells, particularly lymphocytes (T cells and B cells). It is itself activated by the rise in intracellular concentration of calcium ions that follows stimulation of receptors on the cell surface. Calcineurin is inhibited by the drugs ciclosporin and tacrolimus, which is the basis for their use as immunosuppressants to inhibit T-cell activity, for example in preventing rejection of tissue grafts.