Biotechnology is responsible for a large increase in the number of approved medicines called "protein drugs," and for an even larger increase in the number of protein therapeutics now being used in clinical trials. Many human therapeutics, such as monoclonal antibodies used to treat cancer, require correct configuration and animal-specific modifications to be effective. This means that the production of many human protein drugs cannot be carried out in bacteria or plants, but rather is confined to the cells of mammals. Mammalian cell culture, however, is very expensive and low yields limit the amount and number of different proteins that can be developed. Consequently, researchers are attempting to address this problem by producing therapeutic proteins in the milk of domestic farm animals. The major function of the milk-producing mammary gland is to produce proteins. Transgenic animals - that is animals carrying an additional segment of DNA encoding the therapeutic protein - produce this one additional protein in their milk. These animals and their milk are not intended to enter the food supply, but rather to produce these potentially life-saving proteins in their milk. With hundreds of protein therapeutics currently in clinical trials, transgenic animals may become an important source of these protein drugs as they become available to the patient.