insulin consists of two short polypeptide chains: chain A and chain B, that are linked together by disulphide bridges . In mammals, insulin is synthesised as a pro-hormone which contains an extra stretch called the C peptide . This C peptide is not present in the mature insulin and is removed during maturation into insulin . Genetically engineered insulin (e.g., by Eli Lilly) involves producing chains A and B separately and combining them, meaning the C peptide is absent in the therapeutic product . Therefore, statements referring to the absence of C-peptide in mature insulin, the presence of C-peptide in pro-insulin, and the disulphide linkage are correct.