Abstract: Described is a method of producing a soluble bioactive domain of a protein, the method comprising the step of selecting suitable soluble subunits of a protein and assessing the produced protein for desired activity. The method may comprise the steps of amplifying DNA encoding at least one candidate soluble domain, cloning the amplified DNA into at least one expression vector, using each of said vectors into which the DNA has been cloned to each transfect or transform one or more host cell strains, expressing said DNA in one or more host cell strains, and analyzing expression products from said host cells for solubility.