Rennet (pronounce /ˈrɛnɪt/) is a natural complex of enzymes produced in any mammalian stomach to digest the mother's milk, and often used in the production of cheese. Rennet contains a proteolytic enzyme (protease) that coagulates the milk, causing it to separate into solids (curds) and liquid (whey). The active enzyme in rennet is called chymosin or rennin (EC 3.4.23.4) but there are also other important enzymes in it, e.g., pepsin or lipase. There are non-animal sources for rennet substitutes.