Prior to genetic engineering, most cheeses were made with animal-based rennet, an enzyme that causes milk to form curds. The most abundant source for rennet is in the stomach of a nursing calf. The only way to get to this source is to slice the baby cow open. The calves do not survive this process. Many cheeses still use animal-based rennet. These days, using genetic engineering, cheese makers can use yeast to provide an animal-free source of rennet. x