Insulin (from the Latin. insula — island) is a protein-peptide hormone produced by β-cells of the islets of Langerhans of the pancreas. Under physiological conditions in β-cells insulin is produced from preproinsulin — RNA the precursor protein consisting of 110 amino acid residues. After transfer across the membrane of the rough endoplasmic reticulum from preproinsulin cleaved signal peptide of 24 amino acids to form proinsulin. A long chain of proinsulin in the Golgi apparatus are packaged into granules, where hydrolysis of tiny four basic amino acid residue with the formation of insulin and C-terminal peptide (a physiological function of C-peptide is unknown).