Insulinomas can be induced in experimental animals by the combined administration of diabetogenic agents with polyadenosine diphosphate (polyADP)-ribose synthetase inhibitors. A complementary DNA (cDNA) library that was constructed from streptozocin-nicotinamide-induced rat insulinomas has been found to contain a novel gene encoding a basic protein of 145 amino acids. The gene was expressed in alloxan-nicotinamide–induced insulinomas as well as in streptozocin-nicotinamide–induced insulinomas but not in normal pancreatic islets or in regenerating islets. This indicates that the activation of the gene designated rig, i.e., rat insulinoma gene, may be a general feature of pancreatic β-cell transformation.