Ribosome-inactivating proteins (RIPs) are toxic N-glycosidases that release a particular adenine located in the universally conserved α-sarcin loop of the 28S RNA of eukaryotic ribosomes (at position 4324 referred to rat) and thus inactivate protein biosynthesis. Most of the known RIPs are produced by plants and can be divided into two groups, type I RIPs that consist only of one protein chain, the A chain, representing catalytic activity, and type II RIPs that contain in addition a cell-binding domain, the B chain. Occasionally, RIPs are also found in bacteria, fungi, algae, and some mammalian tissues, but these RIPs often exhibit other structures than type I or type II RIPs. A group of toxic enzymes from fungi that inactivate ribosomes by cleaving a single phosphodiester bond within the backbone of the 28S ribosomal RNA are sometimes also called RIPs. Ribosome-inactivating proteins are used in a number of attempts as part of targeted toxins in cancer therapy.