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Cyclosporin immunosuppressant complex. Computer model showing a molecule of the immunosuppressant drug cyclosporin (spheres, centre) bound to a cyclophilin protein (white), which in turn is bound to calcineurin (blue). The molecular structure of cyclosporin can also be seen (ball-and-stick, left). The whole process is modulated by calcium ions (green). Cyclosporin reduces the activity of the immune system by interfering with the activity of T cells, and is widely used to reduce the risk of transplant organ rejection. Cyclophilins are a family of proteins from vertebrates and other organisms that bind to cyclosporin. Calcineurin is responsible for allowing the lymphocyte immune response. Here it is blocked, preventing that immune response. (KEYSTONE/SCIENCE PHOTO LIBRARY/RAMON ANDRADE 3DCIENCIA)