Insulin is a protein hormone produced by the Islets of Langerhans which help in regulating blood glucose level in our body. Mature insulin has two peptide chains - chain A and chain B joined by disulfide bonds. The mature insulin is primarily produced as inactive pro-insulin having a chain C between chain A and B. Chain C is removed for activation of the hormone. Genetic engineering process faced the challenge in developing mature insulin and it was overcome by isolating the gene for chain A and chain B and then producing it in the host bacterial cell. Eli Lily, a pharmaceutical company, prepared two separate DNA sequences each making one protein - chain A and chain B which was then produced individually in different host cell. The extracted chains were later combined by creating disulfide bonds giving the mature human insulin.