With the worldwide production of cheese at more than 27 billion pounds annually, the supply of calves is insufficient to meet the demand for rennet by cheese makers. Until 1990, the only source of rennin was calves. Since chymosin is the most important enzyme in rennin for coagulating milk, scientists created a system to make chymosin that doesn't require calves. Using genetic engineering, the gene for chymosin was cut from a calf cell and inserted into the genomes of bacteria and yeast. The microbes make an exact copy of the calf chymosin.
Microbes replicate and grow rapidly, and can be grown continuously.
Thus, the supply of rennet is assured. There are many advantages to the recombinant chymosin. No calves are used to produce rennet. The gene for chymosin was taken from a calf only once. After that, as the microbes replicate, all new cells automatically have a copy of the gene.