Maspin (mammary serine protease inhibitor) is a protein that in humans is encoded by the SERPINB5 gene. This protein belongs to the serpin (serine protease inhibitor) superfamily. SERPINB5 was originally reported to function as a tumor suppressor gene in epithelial cells, suppressing the ability of cancer cells to invade and metastasize to other tissues.

Maspin has been shown in primary breast cancer to be regulated by wild-type p53, defining a new category of molecular targets of p53 that have the potential to negatively regulate tumor invasion and metastasis. Loss of Maspin expression correlates with increased tumor aggressiveness and poor prognosis in advanced breast and prostate cancer. In contrast, Maspin has been shown to be overexpressed in pancreatic, ovarian, thyroid, gastric, lung, bladder, breast, skin and colon cancer. Several studies have investigated the prognostic significance of Maspin expression in lung cancer. In primary non-small cell lung cancer (NSCLC) was an independent negative prognostic factor for overall survival, whereas strong nuclear Maspin expression was associated with increased overall and disease-free survival in patients with resectable NSCLC.