Rapamycin immunosuppressant drug molecule. Electron-density model of a molecule of the immunosuppressant drug rapamycin, also known as sirolimus. It is mainly used to prevent rejection in organ transplantation, and is used in kidney transplants. It also has anti-fungal and anti-cancer properties, and was originally derived from soil bacteria found on Easter Island. The chemical formula is C51.H79.N.O13. The atoms are represented here as colour-coded blobs: carbon (green), hydrogen (white), nitrogen (blue) and oxygen (red).