Matrix metalloproteinases (MMPs) contribute to cell migration and cell proliferation by degrading the extracellular matrix (ECM) and by releasing growth factors bound up in the ECM. The actions of MMPs are held in check by a class of proteins called tissue inhibitors of metalloproteinases (TIMPs). Vu and Werb review the physiological functions of MMP in development and in cell regulation and cover in more depth the topics ranging from bone and mammary development to zygotic implantation and wound healing. MMPs may act independently or in concert with other MMPs that share some redundant actions. Thus, the authors point out, the elucidation of the functions that each MMP has within organismic development and, more narrowly, organ development, have only been revealed through difficult work. Similarly, what the favored substrate is for each MMP is largely unknown. Vu, T.H., and Werb, Z. (2000) Matrix metalloproteinases: Effectors of development and normal physiology. Genes Dev . 14 : 2123-2133. [Full Text]