– Serine protease inhibitor A3N(Serpin A3N) is a serine protease inhibitor that is structurally related to α1 antichymotrypsin encoded by the SERPINA3 gene. Serpin A3N is highly expressed in brain, testis, lung, thymus, and spleen. It is expressed with low levels in bone marrow, kidney and skeletal muscle. Serpin A3N secreted by Sertoli cells may regulate the activity of locally produced Granzyme B. Granzyme B inhibition by Serpin A3N may therefore regulate Granzyme Bmediated killing by cytotoxic lymphocytes, providing a means to disable cellmediated immune responses.