Cystine bonds are formed in Proinsulin-1 between cysteine residues 31 and 96, cysteine residues 43 and 109, and cysteine residues 95 and 100. Ero1-like alpha does not directly catalyze the oxidation of cysteines to cystine. Instead it maintains a suitably oxidizing environment for the reactions to occur . Though Ero1-like alpha can act via specific isomerases such as P4HB/PDI, there is currently no evidence that formation of cystine bonds in insulin requires a specific isomerase. Interestingly, even in beta cells of wild type animals, trace amounts of incorrectly bonded proinsulin can be detected. Thus, the formation of correct cystine bonds may involve a period of bond shuffling.