Endoproteolytic processing of the human protein C (HPC) precursor to its mature form involves cleavage of the propeptide after amino acids Lys-2-Arg-1 and removal of a Lys156-Arg157 dipeptide connecting the light and heavy chains. This processing was inefficient in the mammary gland of transgenic mice and pigs. We hypothesized that the protein processing capacity of specific animal organs may be improved by the coexpression of selected processing enzymes. We tested this by targeting expression of the human proprotein processing enzyme, named paired basic amino acid cleaving enzyme (PACE)/furin, or an enzymatically inactive mutant, PACEM, to the mouse mammary gland. In contrast to mice expressing HPC alone, or to HPC/PACEM bigenic mice, coexpression of PACE with HPC resulted in efficient conversion of the precursor to mature protein, with cleavage at the appropriate sites. These results suggest the involvement of PACE in the processing of HPC in vivo and represent an example of the engineering of animal organs into bioreactors with enhanced protein processing capacity.