Pyrenophora tritici-repentis (Ptr) is a fungal pathogen that causes tan spot of wheat (Triticum aestivum) in sensitive cultivars. This disease is associated with the production of host-selective toxins. Members of this superfamily share the structure of ToxA, the first discovered proteinaceous host-selective toxin produced by certain Ptr races and which is necessary and sufficient to cause cell death in sensitive wheat cultivars. ToxA adopts a single-domain, beta-sandwich fold of novel topology and its action, which involves several recognition events, is mediated by an important solvent-exposed Arg-Gly-Asp (RGD) containing loop. This structure is remarkably similar to the classic mammalian RGD-containing domain, the fibronectin type III (FnIII) domain. Both the similar topologies and the positional conservation of the RGD-containing loop suggest that ToxA is a distant relative of mammalian FnIII proteins and that to gain entry it binds to an integrin-like receptor in the plant host PMID:16214901.