This represents the eighth slide in a series highlighting the history, chemistry, and uses of ricin, a highly toxic poison made from castor beans. The slide, entitled, “Pathogenesis”, describes how the ricin molecule affects a cell. First, the toxin enters the cell by way of endocytosis, through a coated pit on the cell’s surface. Secondly, after being shuttled to an endosome, the ricin is delivered to the Golgi apparatus, from which it makes its way to the cellular cytosol, where it begins its deactivation of ribosomes. “One molecule of ricin is capable of inactivating 1500 ribosomes/minute, thereby, killing the cell. This presentation is copyrighted by Dr. Jonathan Burstein, MD, Anthony J. Carbone, MD, MPH, of the Harvard Center for Public Health Preparedness, Harvard School of Public Health.