An isolated heteromultimer comprising an immunoglobulin heterodimer Fc region comprising a first monomeric Fc polypeptide and a second monomeric Fc polypeptide and at least one single domain antigen-binding construct attached to one monomeric Fc polypeptide, wherein the isolated heteromultimer is devoid of immunoglobulin light chains; wherein the heterodimer Fc region comprises a variant CH3 region comprising amino acid mutations that promote the formation of the heterodimer Fc region as compared to a homodimeric Fc region, the amino acid mutations comprising: (a) an amino acid mutation at position F405 and the amino acid mutations L351Y and Y407V in the first monomeric Fc polypeptide, and the amino acid mutation T394W in the second monomeric Fc polypeptide, wherein the amino acid mutation at position F405 is F405A, F405I, F405M, F405T, F405S, F405V or F405W, or (b) amino acid mutations F405A and Y407V in the first monomeric Fc polypeptide, and amino acid mutations T366L or T366I and T394W in the second monomeric Fc polypeptide, or (c) amino acid mutations L351Y, F405A and Y407V in the first monomeric Fc polypeptide, and an amino acid mutation at position T366 and the amino acid mutation T394W in the second monomeric Fc polypeptide, wherein the amino acid mutation at position T366 is T366A, T366I, T366L, T366M, T366Y, T366S, T366C, T366V or T366W; or (d) the amino acid mutation Y407A in the first monomeric Fc polypeptide, and the amino acid mutation K409F and one of the amino acid mutations T366A, T366C, T366L, T366M or T366S in the second monomeric Fc polypeptide; (e) the amino acid mutations L351Y and Y407A in the first monomeric Fc polypeptide, and the amino acid mutation K409F and one of the amino acid mutations T366A or T366V in the second monomeric Fc polypeptide; or (f) the amino acid mutations L351F and Y407A in the first monomeric Fc polypeptide, and the amino acid mutations T366A and K409F in the second monomeric Fc polypeptide; wherein the heterodimer Fc region is based on a human type G immunoglobulin (IgG); wherein the variant CH3 domain has a melting temperature (Tm) of 74°C or greater, and the heterodimer Fc region has a purity greater than about 90%; wherein the single domain antigen-binding construct is a single domain antibody (sdAb), a camelid immunoglobulin single variable domain (VhH), a cartilaginous fish antibody fragment (V NAR), an SH3-derived fynomer or a fibronectin-derived binding domain, and wherein the numbering of amino acids is according to the EU index as set forth in Kabat. (a) an amino acid mutation at position F405 and the amino acid mutations L351Y and Y407V in the first monomeric Fc polypeptide, and the amino acid mutation T394W in the second monomeric Fc polypeptide, wherein the amino acid mutation at position F405 is F405A, F405I, F405M, F405T, F405S, F405V or F405W, or (b) amino acid mutations F405A and Y407V in the first monomeric Fc polypeptide, and amino acid mutations T366L or T366I and T394W in the second monomeric Fc polypeptide, or (c) amino acid mutations L351Y, F405A and Y407V in the first monomeric Fc polypeptide, and an amino acid mutation at position T366 and the amino acid mutation T394W in the second monomeric Fc polypeptide, wherein the amino acid mutation at position T366 is T366A, T366I, T366L, T366M, T366Y, T366S, T366C, T366V or T366W; or (d) the amino acid mutation Y407A in the first monomeric Fc polypeptide, and the amino acid mutation K409F and one of the amino acid mutations T366A, T366C, T366L, T366M or T366S in the second monomeric Fc polypeptide; (e) the amino acid mutations L351Y and Y407A in the first monomeric Fc polypeptide, and the amino acid mutation K409F and one of the amino acid mutations T366A or T366V in the second monomeric Fc polypeptide; or (f) the amino acid mutations L351F and Y407A in the first monomeric Fc polypeptide, and the amino acid mutations T366A and K409F in the second monomeric Fc polypeptide;