Insulin is,naturally secreted by pancreas through ffcells of islets of langerhence. Insulin control the blood glucose level. When pancreas failed to produce enough insulin, it causes diabetes. This problem could only be controlled by giving insulin to the body. Initially, insulin gives to a diabetic patient wax extracted from pancreas by killing animals such as cow and pig. This insulin injected to human create allergies other kind of problems. Naturally synthesised pro - insulin made by mammals contain a C - peptide that insulin undergoes maturation and C - peptide separate out when blood glucose level increases. When insulin was produced using rDNA techniques, this separation of C - peptide become a major challange to get mature insulin artificially.