Ricin is an abundant protein component of Ricinus communis seeds (castor beans) that is exquisitely toxic to mammalian cells. It consists of an enzymic polypeptide that catalyzes the N-glycosidic cleavage of a specific adenine residue from 28S ribosomal RNA, joined by a single disulfide bond to a galactose (cell)-binding lectin. The enzymatic activity renders ribosomes containing depurinated 28S RNA incapable of protein synthesis. The bipartite molecular structure of ricin allows it to bind to the mammalian cell surface, enter via endocytic uptake, and deliver the catalytically active polypeptide into the cell cytosol where it irreversibly inhibits protein synthesis causing cell death. Because of its cytotoxic potency, modified ricin is being used for the selective killing of unwanted cells and for the toxigenic ablation of cell lineages in transgenic organisms.