Maspin (mammary serpin) is a novel serine protease inhibitor related to the serpin family with a tumor-suppressing function in breast cancer. Maspin was originally identified from normal mammary epithelium by subtractive hybridization and might function as a class II tumor-suppressor gene. Maspin's decreased expression with increased level of malignancy and its loss in metastatic cells is regulated at the transcriptional level. Cytosin methylation and heterochromatinization in the promoter region might account for this down-regulation of maspin. Transfection of tumor cells with maspin cDNA inhibits invasion and motility and decreases tumor growth and metastatic ability in nude mice. Maspin interacts with the p53 tumor-suppressor pathway and function as an inhibitor of angiogenesis in vitro and in vivo. The progressive loss of expression of maspin during tumor progression makes this new protein an interesting diagnostic and prognostic marker. The re-expression of maspin by pharmacological intervention potentially offers a promising approach as a therapeutic option in breast cancer therapy.