Abstract: The invention relates to transgenic non-human multicellular organisms that contain polynucleotides for expressing proteins that alter posttranslational modification. In particular, the invention provides multiply-transgenic animals in which a first transgene encodes a first protein, a second transgene encodes a second protein, and expression of the second protein affects the posttranlational modification of the first protein in cells of said organism. Expression in preferred embodiments is in specific cells and the modified protein is secreted into a bodily fluid. The invention provides related methods, proteins and products. An example provides transgenic animals that express human Protein C and the processing protease PACE/furin in mammary glands and secrete both proteins into milk.