A cDNA encoding a new human matrix metalloproteinase (MMP), tentatively called MMP-23, has been cloned from an ovary cDNA library. This protein exhibits sequence similarity with MMPs, but displays a different domain structure. Thus, MMP-23 lacks a recognizable signal sequence and has a short prodomain, although it contains a single cysteine residue that can be part of the cysteine-switch mechanism operating for maintaining enzyme latency. The C-terminal domain is considerably shortened and shows no sequence similarity to hemopexin, whereas all human MMPs, with the exception of matrilysin, contain four hemopexin-like repeats. Furthermore, MMP-23 is devoid of structural features distinctive of the diverse MMP subclasses, including the specific residues located close to the zinc-binding site in collagenases, the transmembrane domain of membrane-type MMPs, or the fibronectin-like domain of gelatinases. Fluorescent in situ hybridization experiments showed that the human MMP-23 gene maps to 1p36, a location which differs from all MMP genes mapped to date. Recombinant MMP-23 produced in Escherichia coli exhibits low, but significant proteolytic activity against a synthetic substrate commonly used for assaying MMPs. Northern blot analysis demonstrated that MMP-23 is predominantly expressed in ovary, testis, and prostate, suggesting that this new MMP may play a specialized role in reproductive processes.