Ricin molecule. Computer artwork showing the structure of a molecule of the toxic protein ricin. Ricin comprises two entwined amino acid chains; A (yellow) and B (blue). The A-chain is toxic, inhibiting protein synthesis in cells and causing cell death. It is carried into cells by the non-toxic B-chain, which is able to bind to the cell surface. Ricin is derived from the seeds of the castor oil plant (Ricinus communis), making it easy to be used as a toxin by bioterrorists. Less than a milligram is enough to kill an adult. Inhaling ricin causes coughing, pulmonary oedema and death. If ingested, it causes gastroenteritis, liver and kidney damage and death from circulatory failure. There is no antidote.