A human bispecific IgG1 antibody comprising: (a) a modified heavy chain, wherein the CH1 region of the modified heavy chain comprises (i) a substitution of a native cysteine to a non-cysteine amino acid, and (ii) a substitution of a native non-cysteine amino acid to a cysteine amino acid; and (b) a modified corresponding light chain, wherein the CL region of the modified light chain comprises (i) a substitution of a native cysteine to a non-cysteine amino acid, and (ii) a substitution of a native non-cysteine amino acid to a cysteine amino acid; and (c) a second heavy chain and a second corresponding light chain, wherein the second heavy chain and the second corresponding light chain do not comprise a substitution of a native non-cysteine amino acid to a cysteine amino acid and do not comprise a substitution of a native cysteine to a non-cysteine amino acid; and wherein the modified heavy chain and modified corresponding light chain are selected from the group consisting of variants a. comprising a CH1 domain wherein the amino acid at position 141 is substituted for cysteine and the cysteine at position 220 is substituted for a non-cysteine amino acid; and a CL domain wherein the amino acid at position 116 is substituted for cysteine and the cysteine at position 214 is substituted for a non-cysteine amino acid; b. a CH1 domain wherein the amino acid at position 168 is substituted for cysteine and the cysteine at position 220 is substituted for a non-cysteine amino acid; and a CL domain wherein the amino acid at position 164 is substituted for cysteine and the cysteine at position 214 is substituted for a non-cysteine amino acid; c. a CH1 domain wherein the amino acid at position 126 is substituted for cysteine and the cysteine at position 220 is substituted for a non-cysteine amino acid; and a CL domain wherein the amino acid at position 121 is substituted for cysteine and the cysteine at position 214 is substituted for a non-cysteine amino acid; wherein numbering is according to the EU index. (a) a modified heavy chain, wherein the CH1 region of the modified heavy chain comprises (i) a substitution of a native cysteine to a non-cysteine amino acid, and (ii) a substitution of a native non-cysteine amino acid to a cysteine amino acid; and (i) a substitution of a native cysteine to a non-cysteine amino acid, and (ii) a substitution of a native non-cysteine amino acid to a cysteine amino acid; and (b) a modified corresponding light chain, wherein the CL region of the modified light chain comprises (i) a substitution of a native cysteine to a non-cysteine amino acid, and (ii) a substitution of a native non-cysteine amino acid to a cysteine amino acid; and (i) a substitution of a native cysteine to a non-cysteine amino acid, and (ii) a substitution of a native non-cysteine amino acid to a cysteine amino acid; and (c) a second heavy chain and a second corresponding light chain, wherein the second heavy chain and the second corresponding light chain do not comprise a substitution of a native non-cysteine amino acid to a cysteine amino acid and do not comprise a substitution of a native cysteine to a non-cysteine amino acid; and wherein the modified heavy chain and modified corresponding light chain are selected from the group consisting of variants a. comprising a CH1 domain wherein the amino acid at position 141 is substituted for cysteine and the cysteine at position 220 is substituted for a non-cysteine amino acid; and a CL domain wherein the amino acid at position 116 is substituted for cysteine and the cysteine at position 214 is substituted for a non-cysteine amino acid; b. a CH1 domain wherein the amino acid at position 168 is substituted for cysteine and the cysteine at position 220 is substituted for a non-cysteine amino acid; and a CL domain wherein the amino acid at position 164 is substituted for cysteine and the cysteine at position 214 is substituted for a non-cysteine amino acid; c. a CH1 domain wherein the amino acid at position 126 is substituted for cysteine and the cysteine at position 220 is substituted for a non-cysteine amino acid; and a CL domain wherein the amino acid at position 121 is substituted for cysteine and the cysteine at position 214 is substituted for a non-cysteine amino acid; wherein numbering is according to the EU index. a. comprising a CH1 domain wherein the amino acid at position 141 is substituted for cysteine and the cysteine at position 220 is substituted for a non-cysteine amino acid; and a CL domain wherein the amino acid at position 116 is substituted for cysteine and the cysteine at position 214 is substituted for a non-cysteine amino acid; b. a CH1 domain wherein the amino acid at position 168 is substituted for cysteine and the cysteine at position 220 is substituted for a non-cysteine amino acid; and a CL domain wherein the amino acid at position 164 is substituted for cysteine and the cysteine at position 214 is substituted for a non-cysteine amino acid; c. a CH1 domain wherein the amino acid at position 126 is substituted for cysteine and the cysteine at position 220 is substituted for a non-cysteine amino acid; and a CL domain wherein the amino acid at position 121 is substituted for cysteine and the cysteine at position 214 is substituted for a non-cysteine amino acid;