The identification of genes with selective expression in specific organs or cell types provides an entry point for understanding biological processes that occur uniquely within a particular tissue. Using a subtraction approach designed to identify genes preferentially expressed in specific tissues, we have identified prostase, a human serine protease with prostate-restricted expression. The prostase cDNA encodes a putative 254-aa polypeptide with a conserved serine protease catalytic triad and an amino-terminal pre-propeptide sequence, indicating a potential secretory function. The genomic sequence comprises five exons and four introns and contains multiple copies of a chromosome 19q-specific minisatellite repeat. Northern analysis indicates that prostase mRNA is expressed in hormonally responsive normal and neoplastic prostate epithelial tissues, but not in prostate stromal constituents. Prostase shares 35% amino acid identity with prostate-specific antigen (PSA) and 78% identity with the porcine enamel matrix serine proteinase 1, an enzyme involved in enamel matrix degradation and with a putative role in the disruption of intercellular junctions. Radiation-hybrid-panel mapping localized prostase to chromosome 19q13, a region containing several other serine proteases, including protease M, pancreatic/renal kallikrein hK1, and the prostate-specific kallikreins hK2 and hK3 (PSA). The sequence homology between prostase and other well-characterized serine proteases suggests several potential functional roles for the prostase protein that include the degradation of extracellular matrix and the activation of PSA and other proteases.