Stromelysin-3 (ST3) is a putative new matrix metalloproteinase (MMP) which may play a role in the progression of human carcinomas, and exhibits unique structural and functional characteristics among the MMP family. The ST3 gene, which is generally not expressed at significant levels in benign breast tumors, has been found to be expressed in all invasive breast carcinomas tested so far. The gene is also expressed in some in situ breast carcinomas, which have a higher probability to become invasive. ST3 RNA and protein are specifically found in fibroblastic cells immediately surrounding the neoplastic cells, both in invasive and in situ breast carcinomas. The same expression pattern is observed in other types of human carcinomas, and the highest ST3 RNA levels are observed in tumors that exhibit high local invasiveness. The ST3 gene is also expressed in fibroblastic cells during the inflammatory phase of wound healing, which suggests that ST3 gene expression in stromal fibroblasts may be under the control of factors produced by inflammatory cells during wound healing, and by cancer cells during carcinoma progression. ST3 may thus represent a stroma-derived factor necessary for the progression of epithelial malignancies, and its manipulation may possibly be used to develop new anti-cancer agents.