As an important messenger, intracellular free calcium can modulate many cellular functions. Calcium stores, endoplasmic reticulum and sarcoplasmic reticulum, play important functions in controlling the level of intracellular calcium. The activities of calcium release channel on these calcium stores, ryanodine receptor and inositol trisphosphate receptor, are affected by several factors. FK506, an immunosuppressive drug, can specifically bind to a cytosolic receptor with molecular weight of about 12 ku, namely, FK506-binding protein (FKBP12). In vivo, FKBP12 maintains a tight association with calcium release channel and forms a complex. This interaction plays important roles in modulating calcium release channels.