Maspin, or mammary-specific serpin, is a tumor suppressor protein of 42 kD that belongs to the serine proteinase inhibitor (serpin) family. It is reported to be expressed in normal breast and prostatic epithelial cells but is downregulated in carcinomas derived from these cell types. The expression of maspin is controlled at the transcriptional level by a combination of elements including Ets, AP-1 and p53. The tumor suppressor activity of maspin may depend on its ability to inhibit angiogenesis. In breast myoepithelial cells, maspin is predominantly a soluble cytoplasmic protein which associates with secretory vesicles and is present at the cell surface. The loss of maspin in breast tumors is reported to be a progressive process and expression decreases with increasing malignancy of primary tumors and is absent from lymph node and distant metastases. In rats, maspin mRNA has been detected in mammary gland, vagina, bladder, thymus, small intestine, ventral prostate, seminal vesicles and thyroid, but is absent from heart, lung, liver, brain and kidney.