Proprotein convertases have been proposed to be critical enzymes important in the progression of breast cancer, based on their unique property of generating biologically active growth and receptor molecules from their inactive precursors. In order to understand the biological action of proprotein convertases in the human breast cancer cell line, MCF-7 was stably transfected with a CMV-mPC1/neo construct. Eight G418 resistant clones were tested by northern blot and Southern blot; only clone 2 and clone 6 were found to express mPC1 mRNA. Clone 6, as a higher expressor of mPC1 mRNA level compared to clone 2, was labeled with $\sp{35}$S-Cysteine. Twenty-four hours after labeling, both cell lysate and medium were collected. Immunoprecipitation with rabbit polyclonal anti-PC1 antibody followed by one- and two-dimensional SDS-polyacrylamide gel electrophoresis was carried out, to confirm that an elevated level of mPC1 protein was produced in the stable transfected cell line. In order to identify potential PC1 target proteins in human breast cancer, two-dimensional gel analysis was used to compare cellular proteins between clone 6 and clone 7; the latter was a G418 resistant clone but did not contain an integrated mPC1 construct. After four separate experiments, four consistent and reproducible proteins were identified which differed between clone 7 and clone 6. One or more of these proteins may be a candidate for PC1 cleavage in human breast cancer. Further analysis of these protein spots is necessary to gain insights into the identity and potential relationship between these proteins.