The genetic sequence of insulin was worked out early as a result of figuring out the amino acid sequence, then working back to the genetic code. When this code was known, it was then possible to take advantage of recombinant DNA technology to insert the gene into a bacterial cell.
When insulin from other living things is examined, it is apparent that all insulin molecules are not equivalent. When looking for human insulin substitutes to inject into humans, cow and pig insulins were found to only differ by three and one amino acid respectively. These insulins were purified and used as "semisyntheic" or "humanized" insulin. The supply of this insulin was insufficient, it was expensive, and difficult to purify. Additionally, while the insulin initially functioned properly, patients' immune systems would frequently produce antibodies against the foreign insulin.
The development of recombinant human insulin in the 1980's has been a major improvement to the treatment of diabetes. The gene that produces insulin is located on chromosome 11 of humans and was successfully inserted into the K-12 strain of
E. coli
bacteria.