A class I ribosome-inactivating protein (RIP) and toxin derived from the seeds of the plant Gelonium multiflorum. Gelonin (rGel) exerts N-glycosidase activity on the 28S ribosomal RNA (rRNA) unit of eukaryotic ribosomes by cleaving out adenine at the 4324 site, which depurinates rRNA, inactivates ribosomes, inhibits protein synthesis, and results in cell death. Used as the toxin moiety of certain immunotoxins and linked to antibodies specific for a tumor-associated antigen (TAA), gelonin can be targeted to and induce specific cytotoxicity in tumor cells expressing the TAA.