Insulin is a protein hormone that is composed of two chains, an A-chain having 21 amino acids and a B-chain having 30 amino acids, linked together by disulphide bonds (S-S). Insulin is derived from a 74−amino acid prohormone molecule called proinsulin having C-peptide and this is removed before the processing of mature insulin and during the production of recombinant insulin, the two chains were separately produced and then linked with each other. C-peptide is absent in mature insulin.