Insulin is synthesised as a prohormone which contains an extra stretch called C peptide. This C peptide is not present in the mature insulin as it is removed during maturation. The main challenge for insulin production using rDNA technique was getting insulin assembled into a mature form. In 1983, Eli Lily, an American company, first prepared two DNA sequences corresponding to $A$ and $B$ chains of human insulin and introduced them in plasmids of $E$. coli to produce insulin chains. Chain A and B were produced separately, extracted and combined by creating disulphide bonds to form human insulin.