Throughout most of the 20th century, millions of diabetics worldwide depended on insulin extracted from pigs or cows. Unfortunately, only so much insulin can be extracted from these animals, and some diabetics developed allergic reactions to cow and/or pig insulin. The availability of human insulin in unlimited quantities would certainly have a marketable value.
Insulin itself is a small protein, and was the first to have its amino acid sequence and structure determined. Insulin is made up of two polypeptide chains. The A chain has 21 amino acids and the B chain has 30 amino acids. With so much known about it, many researchers thought that insulin should be easy to make in a laboratory.