ST14 and breast cancer: In order to begin to explore whether matriptase proteolytic activity associated with breast cancer cells might be used as a target for the development of drugs and/or imaging probes, we measured the cleavage of the substrate Boc-Gln-Ala-Arg-AMC by MCF7 breast cancer cells prior to and following the induction of robust matriptase zymogen activation (Fig. 1A).