FK-506 is a potent immunosuppressant and in vitro T cell proliferation blocker. It has been shown to disrupt calcineurin (also known as CaN or phosphatase 2B)-mediated signal transduction in T lymphocytes. The compound interacts with its FK506-binding protein-12 (FKBP12). The resulting complex, in turn, interferes with calcineurin substrate interaction.