Bovine somatotropin (bST) is a growth hormone found in cattle; versions of this hormone are found in all mammals, including humans.
Injections of this hormone dramatically increase the milk production of lactating cows. In past years, the hormone was very expensive because it could only be taken from slaughtered cows, but in the early '90s researchers learned how to genetically engineer the bacterium E. coli to produce it and make it vastly cheaper. Now, many dairy producers use the hormone, but the practice is controversial because the use of bST may increase the incidence of mastitis (udder infection) in cows, and the long-term human health effects of the slightly increased hormone levels in the milk from treated cows have not been established.
From the science dictionary at http://biotech.icmb.utexas.edu/