A non-human transgenic mammalian animal, as described above, contains one or more exogenous double stranded DNA sequence(s) stably integrated into the genome of the animal, which comprises trans-acting regulatory units controlling expression of DNA sequences encoding proteins to be secreted into the milk of transgenic mammals. The DNA sequence of the trans-regulatory gene encodes transcriptional activating proteins, which are not secreted but made in a temporally controlled and mammary tissue specific manner. The DNA sequence containing the protein to be secreted in the milk is constructed on a separate gene sequence under the regulation of a minimal promoter and a trans-activation binding domain. The transgenic mammals are preferably pigs, cows, sheep, goats and rabbits. A related composition and method for making transgenic proteins which require specialized propeptides for proper post-translational processing is also described.