Genetic engineering of milk composition: modification of milk components in lactating transgenic animals.

Recent progress in recombinant DNA technology as well as in embryo manipulation and transfer has made the introduction of specific genes into the germline of animals relatively commonplace. With appropriate genetic constructs expression of the inserted genes in transgenic animals can be controlled in a tissue-specific and in a differentiation-specific manner; thus, it is now possible to consider alteration of the composition of milk produced by a lactating animal in any of a variety of ways. There is a growing list of foreign milk proteins that have been expressed, and one can envisage placing almost any protein gene of interest under the control of the cis-acting promoter and enhancer elements of a milk protein gene. Modification of milk composition can be extended not only to the proteins of commodity value but also, by manipulation of key metabolic enzymes, to fat, lactose, and other minerals in milk.