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In the field of structural biology it is easy to forget that the majority of protein structures determined to date help to explain only part of cellular protein function. The determination of protein structures to atomic resolution currently requires proteins to be water soluble, and to either crystallise (for X-ray crystallography) or be smaller than around 200 residues (for NMR spectroscopy). Many important structural proteins in the extracellular matrix are insoluble: collagen, elastin and keratin, for example. These are classed as fibrous proteins. The phospholipid cell, endoplasmic and nuclear membranes are foci for the majority of cellular events. The membrane provides a medium for the co-localisation of proteins, some of which span the membrane one or more times. However, the folding, structure and stability of membrane spanning proteins are generally dependent on the membrane environment and this makes them difficult to solubilise intact for structure determination. A handful of membrane proteins have been crystallised however, including the photosynthetic reaction centre of Rhodopseudomonas viridis[Deisenhofer et al., 1985], a purple sulphur bacterium, and porin, the outer membrane channel protein of mitochondria[Weiss & Schulz, 1992].