Abstract: Disclosed is a method for expressing a heterologous gene in the mammary gland of a ruminant mammal. The invention is useful for producing a heterologous protein(s) in the milk of the mammal, for producing a heterologous enzyme(s) which is tethered to the mammary epithelial cell membrane and which can act on a component of the milk of the mammal, and for providing an antisense oligonucleotide(s) or ribozyme(s) which inhibits expression of a gene to a mammary epithelial cell. The method involves infusing a liquid complex including a genetic construct into a ductal tree of the mammal. The liquid complex can be infused before the mammal reaches sexual maturity and after the mammal develops a functional streak canal. Alternatively, a liquid complex that is free of live retroviruses can be infused. If desired, the infused genetic construct can be treated with a polycationic compound and/or a lipid to improve the efficiency with which it is taken up by an epithelial cell of the mammary gland.