A mouse fusion protein comprising two chimeric polypeptide chains each comprising two murine IgG heavy chain chains having an immunoglobulin tailpiece from, or derived from, a non-native immunoglobulin and wherein the amino acid sequence of each of the IgG heavy chain constant regions comprises an amino acid modification (e.g. M84C) which promotes the multimerisation or polymerisation of the fusion protein. Preferably the tailpiece is human and is derived from IgM or IgA and has a substitution or deletion at residue C17. Preferably, each chimeric chain comprises one or more murine hinge regions and a targeting moiety such as an ntigen binding region, small molecule or nucleic acid. The invention further relates to the uses of the fusion protein to detect a target molecule in a sample, preferably as a means for diagnosis such as blood group haemagglutination.
