The invention relates a fragment derived from the MET ectodomain which the inventors have found is capable, in monomer form, of binding to HGF/SF either in the presence or absence of heparin. The availability of soluble, monomeric forms of the MET receptor enabled studies of its solution properties and HGF/SF binding and provides an assay comprising the steps of (a) providing a MET ectodomain fragment; (b) providing an agent; and (c) determining the extent to which the agent interacts with said fragment.