What it is
The enzyme that turns milk into cheese — traditionally from a calf's stomach, now often the identical enzyme brewed by microbes.
Why your biology objects
Chymosin coagulates casein, the foundational step of cheesemaking — animal rennet is the literal ancestral coagulant (cheese was set in stomach-bag vessels). The fermentation-produced version is molecularly identical to calf chymosin and demonstrably safe, just made by an engineered microbe — a biotech route, not a harm.