Reduced cardiac scarring in MMP-9-deficient mice (See article on pages 55-62)Healing of injured tissue typically occurs in several stages, beginning with inflammation, in which collagen and other extracellular matrix (ECM) proteins are degraded, and continuing with synthesis and deposition of new tissue and remodeling of the local ECM.Matrix metalloproteinases (MMPs) may act at each of these stages, but precise roles for individual secreted proteinases are only now emerging.Ducharme and colleagues have followed the histological changes in the left ventricle following induced myocardial infarction (MI), comparing MMP-9-deficient mice with their wild-type littermates.The earliest phases of repair occurred normally in MMP9 -/-mice, but later in the process, collagen deposits were poorly organized and relatively scarce in the absence of the proteinase.Because collagen I mRNA is present at wild-type levels, the authors suggest that collagen turns over more rapidly, perhaps as a result of increased expression of several other MMPs in infarcted hearts of MMP9 -/-mice.Ducharme et al. suggest that inhibition of MMP-9 might prove useful in preventing scarring after MI.