Abstract: Disclosed are binding molecules which are recombinant polypeptides containing: (i) a binding domain capable of binding a target molecule, and (ii) an effector domain having an amino acid sequence substantially homologous to all or part of a constant domain of a human immunoglobulin heavy chain; characterized in that the binding molecule is capable of binding the target molecule without triggering significant complement dependent lysis, or cell mediated destruction of the target, and more preferably wherein the effector domain is capable of specifically binding FcRn and/or Fc?RIIb. These are generally based on chimeric domains which are derived from two or more human immunoglobulin heavy chain CH2 domains domains. In preferred embodiments the regions 233-236, and 327-331, are modified, as are further residues to render the molecule null allotypic. Also disclosed are nucleic acids, host cells, production processes and materials, and uses. Pharmaceutical preparations are also disclosed.