Maspin is a 42 kDa tumor suppressor protein that belongs to the serine protease inhibitor (serpin) family. It inhibits cell motility and invasion in vitro. and tumor growth and metastasis in nude mice: however. maspin's molecular mechanism of action has remained elusive. Maspin contains several tyrosine residues and we hypothesized that phosphorylation of maspin could play a role in its biological function. Our study reveals that maspin is phosphorylated on tyrosine moiety (ies) in normal mammary epithelial cells endogenously expressing maspin. In addition. transfection of the maspin gene. using either a stable or inducible system into maspin-deficient breast cancer cell lines. yields a protein product that is phosphorylated on tyrosine residue(s). Furthermore, recombinant maspin protein can be tyrosine-phosphorylated by the kinase domain from the epidermal growth factor receptor in vitro. These novel observations suggest that maspin., which deviates from the classical set-pin. may be an important signal transduction molecule in its phosphorylated form (C) 2002 Elsevier Science (USA). All rights reserved.