Insulin obtained from the pancreas of cattle and pigs slightly differ from the human insulin in their amino acid sequence. Moreover, the insulin production from pig and cattle is not sufficient to cater the needs of growing number of diabetic patients. Also, the injection of insulin into the patients, occasionally produces sensitivity reaction and side effects. These factors led researches to look for some alternative source of human insulin. The search for new source was soon fulfilled by recombinant $DNA$ technology