Abstract: Proteinaceous products can be produced by transgenic animals having genetic constructs integrated into their genome. The construct comprises a 5′-flanking sequence from a mammalian milk protein gene (such as beta-lactoglobulin) and DNA coding for a heterologous protein other than the milk protein (for example a serin protease such as alpha1-antitrypsin or a blood factor such as Factor VIII or IX). The protein-coding DNA comprises at least one, but not all, of the introns naturally occurring in a gene coding for the heterologous protein. The 5′-flanking sequence is sufficient to drive expression of the heterologous protein.

Abstract: A method for purifying human or other alpha-l proteinase inhibitor (&agr;1-PI) from a solution (which may be derived from the milk of a transgenic animal expressing the &agr;1-PI) which comprises contacting the solution with a cation exchange substrate under conditions sufficient to bind non-tg-&agr;1-PI contaminants to the substrate while not substantially binding tg &agr;1-PI to the substrate. Using the preferred embodiment, the purified tg &agr;1-PI contains as little as 40 pg non-&agr;1-PI-whey protein per mg total protein.