Provided herein are isolated heteromultimers comprising: at least one single domain antigen-binding construct attached to at least one monomer of a heterodimer Fc region wherein the heterodimer Fc region comprises a variant CH3 domain comprising amino acid mutations that promote the formation of said heterodimer with stability comparable to that of a native Fc homodimer and wherein said isolated heteromultimer is devoid of immunoglobulin light chains and optionally devoid of immunoglobulin CH1 region. These novel molecules comprise complexes of heterogeneous components designed to alter the natural way antibodies behave and that find use in therapeutics.
