Ciclosporin, also spelled cyclosporine, is an immunosuppressant drug widely used in organ transplantation to prevent rejection. It reduces the activity of the immune system by interfering with the activity and growth of T cells. It was initially isolated from the fungus Tolypocladium inflatum found in a soil samples in 1969 from Norway, by Hans Peter Frey. Most peptides are synthesized by ribosomes, but ciclosporin is a cyclic nonribosomal peptide of 11 amino acids and contains a single D-amino acid, which is rarely encountered in nature. It is on the WHO Model List of Essential Medicines, the most important medications needed in a basic health system.