The invention provides multivalent ligand binding agents (traps) for members of the TGF-β superfamily and polypeptide linkers and methods for making and using such constructs. The traps may be used as therapeutic or diagnostic (imaging or non-imaging) agents for diseases/disorders caused by over-production/activity of the target ligand. In an embodiment of the invention there is provided a multivalent binding agent with affinity for a member of the TGF-β superfamily, the agent having the general structure I: ( -linker1)k-[{ -linker2- -linker3f-}n-( )m-(linker4- )d]h, where: n and h are independently greater than or equal to 1; d, f, m and k are independently equal to or greater than zero; bd's are polypeptide binding domains having an affinity for the same member of the TGF-β superfamily; and, linkers are unstructured polypeptide sequences.