Status of research with transgenic farm animals.

Microinjection is the predominant method used to transfer genes into farm animals. Current research is devoted to improvement of productivity traits, enhancement of animal health, and production of biomedically useful human health products. Initial transgenic research primarily involved genes coding for growth hormone (GH), growth hormone-releasing factor (GRF), and insulin-like growth factor I (IGF-I). More recent investigations have attempted to stimulate muscle development, to use bacterial enzymes so animals can synthesize certain essential amino acids, to induce expression of specific immunoglobulin or disease-resistance genes, and to direct expression of human proteins to the mammary gland, specific organs, or specific cells for production of useful human health products. The main limitations to progress are the lack of useful cloned genes for productivity traits and disease resistance and the insufficient knowledge of mechanisms involved in regulation of transgenes.