Interest in modifying traits that determine productivity of domestic animals was greatly stimulated by early experiments in which body size and growth rates were dramatically affected in transgenic mice expressing growth hormone transgenes driven by a metallothionein (MT) enhancer/promoter. From that starting point, similar attempts followed to enhance growth in farm animals by introduction of various growth factors, modulators, and their receptors. It soon became apparent that transgene regulation was an exquisite balancing act, where precise regulation of transgenes was crucial to normal development. Yet, the overexpression of various transgene products illustrated that such animals could produce biologically important molecules as efficient mammalian bioreactors, with efficiencies far greater than conventional bacterial or cell culture systems. From early studies in the mid-1980s through the 1990s, one of the main targets of genetic engineering or gene pharming efforts has involved attempts to direct expression of transgenes encoding biologically active human proteins in farm animals. To date, expression of foreign genes encoding various protein products was successfully targeted to the mammary glands of goats, sheep, cattle, and swine, yet the jump from model to achieving regulatory approval has proven most challenging.

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