FK506, current lot 99.9 pure, by Fermentek is 23-membered macrolide lactone discovered in 1984 exhibits potent inhibitory effect on T- lymphocyte activation. It binds to immunophilins FK506 binding proteins (FKBP-12) and a complex of FKBP-12, calcium, calmodulin and calcineurin is formed, inhibiting phosphatase activity of calcineurin.