Rennin, also known as Chymosin, is a pepsin-related proteolytic enzyme synthesized by cells in the stomach of certain animals that efficiently converts liquid milk into a semi-solid, allowing it to remain in the stomach for longer. The natural substrate of Rennin is K-casein, which is specifically cleaved at the peptide bond between amino acid residues 105 and 106, phenylalanine and methionine, and is widely used in cheese production.