In this issue of the BJO an interesting study is presented by Salzmann et al (p 1092) in which they analyse, by immunohistochemistry, the epiretinal membranes in proliferative diabetic retinopathy (PDR) for the presence of certain matrix metalloproteinases (MMPs) and their inhibitors. MMPs are a group of zinc dependent proteolytic enzymes that play an important part in the degradation of extracellular matrix components during developmental, physiological, and pathological processes. These enzymes are naturally inhibited in the extracellular matrix by the tissue inhibitors of metalloproteinases (TIMPs), and expression of MMPs and TIMPs is regulated by a variety of growth factors, cytokines, oncogenes, and tumour promoters. Currently, at least 18 MMPs and four TIMPs have been identified, and their potential roles in vitreoretinal disorders have been discussed recently in aBJO perspective.1

MMPs are very likely to have an important role in the pathological processes underlying PDR. However, with such a complex system of enzymes and enzyme regulation, a tenet that is central to the work of Salzmann et al is that certain MMPs or TIMPs play a specific part in PDR; if these could be identified, then it may be possible to selectively modulate their actions in order to improve the treatment of this condition. Salzmann et al looked for the presence of MMP-1, MMP-2, MMP-3, …