The present disclosure relates to dimers of engineered polypeptides having a binding affinity for the neonatal Fc receptor FcRn, and provides an FcRn binding dimer, comprising a first monomer unit, a second monomer unit and an amino acid linker, wherein the first and second monomer units each comprises an FcRn binding motif. The FcRn binding dimer binds FcRn with higher capacity compared to the first monomer unit or second monomer unit alone. The present disclosure also relates to the use of the FcRn binding dimer as an agent for modifying pharmacokinetic and pharmacodynamic properties and as a therapeutic agent.