The family of enzymes known as serine proteases supports many biological functions for cancer cells, including activation of growth and angiogenic factors and activation of other proteases for invasion and metastasis. In addition, many of these serine proteases are secreted by cells into the extracellular space to serve these functions. Therefore, serine proteases are excellent candidate tumor markers. To examine serine proteases expressed by ovarian carcinoma, we designed degenerate PCR primers corresponding to the conserved regions of these genes and used them in reverse transcriptase-PCR experiments with normal and tumor cDNA as a template. The PCR products were subcloned and sequenced, and one of these clones was found to encode a novel serine protease, named tumor-associated differentially expressed gene-14 (TADG14). Northern blot analysis indicated that the mRNA for TADG14 is 1.4 kb long and that it is highly overexpressed in ovarian carcinoma compared with normal ovary. The entire cDNA has been obtained, and based on sequence homology, it encodes a 260-amino acid serine protease. Semiquantitative PCR indicates that TADG14 is overexpressed in 24 of 40 tumors studied. Northern blot data confirm this overexpression, and immunohistochemical staining suggests that this protein is secreted. As such, the TADG14 protease may be useful as a diagnostic tool or as a molecular target for therapy.