Chymosin
The enzyme in rennet that makes milk set, now mostly made by fermentation rather than taken from calves.
Also: rennin, FPC, fermentation-produced chymosin
Chymosin is the enzyme that does most of rennet's work. It makes a single precise cut in kappa-casein, the outer layer of the milk's casein clusters, after which the clusters bind together into curd. Young calves make it in the stomach to curdle their mother's milk. Since 1990, chymosin with the same structure has been made by fermentation, using fungi or yeasts that carry the gene for it, and this fermentation-produced chymosin, or FPC, now sets a large share of the world's cheese. Because it is not taken from animals, cheese made with it is usually accepted as vegetarian. Protected cheeses such as Parmigiano Reggiano still insist on calf rennet.