Chymosin (or rennin)'s an aspartic acid protease enzyme found in rennet. It's produced by the cow, in the lining of the abomasum (its fourth stomach). Bovine chymosin's produced nowadays recombinantly in E.coli, Aspergillus niger var awamori, and K.lactis as alternative resource to the one from the cows. The gene's found in humans (on chromosome 1), but it isn't in a functioning form. Rennin's produced by gastric chief cells in infants to curdle the milk they ingest, allowing a longer residence… (More on Rennin)