For example, although microorganisms such as bacteria generate many drugs, sometimes drug proteins useful to humans require changes to their structure after they have been synthesized. Bacteria can't be engineered to make these changes, but animal cells can do it. In the past, molecular farming of drugs from transgenic livestock such as pigs, sheep, and cows required the death of the animal so the drug could be harvested from their blood. Human growth hormone and the insulin needed by diabetics are some of the drugs that have been produced this way. Since those early experiments, the focus has shifted to producing useful proteins secreted into the milk of transgenic livestock.
Promoters are the part of a gene that acts as a switch to regulate when a gene product is made.
It is possible to introduce a milk protein promoter connected to a gene coding for a specific drug. Milk protein promoters are tissue specific, meaning they turn on only in the mammary glands, not the blood. Drug production in the blood could make the animal sick, and the drug also could be contaminated by germs present in the animal's system. Getting a drug by milking an animal is more convenient, more humane, and safer than harvesting it from blood. And it works.