A variety of transgenic animal species are being used to produce recombinant proteins. The general approach is to target the expression of the desired protein to the mammary gland using regulatory elements derived from a milk protein gene and then collect and purify the product from milk. Promoter sequences from a number of different milk protein genes have been used to target expression to the mammary gland, although significant problems remain with regard to achieving transgene expression levels consistent with commercial exploitation. The mammary gland appears to be capable of carrying out the complex posttranslational modifications. such as glycosylation and gamma-carboxylation required for the biological activity and stability of specific proteins. Effective purification protocols have been established and products produced by this route have now entered clinical trials.