Matrix metalloproteinases (MMPs) are expressed in normal remodeling tissues in a generally tissue-restricted pattern. Transcripts for stromelysin-1 and collagenase are expressed primarily in stromal fibroblasts, whereas transcripts for matrilysin are expressed primarily in glandular epithelial cells. These expression patterns are maintained at carcinoma tumor sites until the late stages of tumor progression at which point many epithelially-derived tumors begin to express stromal fibroblast MMPs. Coincidentally, late stage carcinomas take on other characteristics of stromal fibroblasts, indicating that these tumor cells have "transdifferentiated', that is, they have begun to exhibit characteristics of cells from a separate developmental lineage. Despite their distinct expression patterns, many of the promoters for MMP genes show the same general arrangement of the nuclear proto-oncoprotein-binding sites, AP-1 and PEA3. However, the specific interaction between these cis-elements and different combinations of Fos, Jun, and Ets proteins which recognize these sites may be important in controlling both the positive and negative regulation involved in the tissue-restricted pattern of MMP expression in normal and neoplastic tissues.