A number of bacterial and plant toxins act by inhibiting protein synthesis in eukaryotic cells. The toxins of the shiga and ricin family inactivate 60S ribosomal subunits by an N-glycosidic cleavage which releases a specific adenine base from the sugar-phosphate backbone of 28S rRNA [<cite idref="PUB00001357"/>, <cite idref="PUB00001175"/>, <cite idref="PUB00000690"/>]. Members of the family include shiga and shiga-like toxins, and type I (e.g. trichosanthin and luffin) and type II (e.g. ricin, agglutinin and abrin) ribosome inactivating proteins (RIPs). All these toxins are structurally related. RIPs have been of considerable interest because of their potential use, conjugated with monoclonal antibodies, as immunotoxins to treat cancers. Further, trichosanthin has been shown to have potent activity against HIV-1-infected T cells and macrophages [<cite idref="PUB00004990"/>]. Elucidation of the structure-function relationships of RIPs has therefore become a major research effort. It is now known that RIPs are structurally related. A conserved glutamic residue has been implicated in the catalytic mechanism [<cite idref="PUB00004654"/>]; this lies near a conserved arginine, which also plays a role in catalysis [<cite idref="PUB00003312"/>].<p> This entry represents a conserved site of the Ribosome-inactivating protein conserved site </p>