Previously we have shown that the type 2 transmembrane serine protease matriptase is a target protease of HAI-2 in breast cancer cells, the exposure of which to a pH 6.0 buffer induces extensive matriptase zymogen activation and the formation of matriptase-HAI-1 and matriptase-HAI-2 complexes [20]. The gene discussed is ST14; the disease is breast carcinoma.