Ricin molecule Computer artwork showing the structure of a molecule of the toxic protein ricin (blue and yellow) with an active ribosome in the background. Ricin comprises two entwined amino acid chains; A (yellow) and B (blue). The A-chain is toxic, inhibiting protein synthesis in ribosomes containing depurinated 28S RNA (ribonucleic acid) 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