A method for selecting a human heavy chain variable region for making a bispecific antibody, comprising: (a) immunizing a genetically modified mouse with an antigen of interest, wherein the mouse comprises (i) a replacement at the endogenous mouse κ immunoglobulin light chain variable region locus of all endogenous mouse κ immunoglobulin light chain variable region gene segments with a rearranged human Vκ1-39/J gene segment, a rearranged human Vκ3-20/J gene segment, or a combination thereof, wherein the rearranged human gene segment is operably linked to an endogenous mouse κ constant gene, wherein the mouse lacks an endogenous mouse κ immunoglobulin light chain variable region locus that is capable of rearranging and forming a gene that encodes a mouse κ variable region; and (ii) a replacement of 90-100% of unrearranged mouse V H gene segments with at least one unrearranged human V H gene segment, and a replacement of all of the endogenous mouse D and J H gene segments with at least one unrearranged human D gene segment and at least one unrearranged human J H gene segment, wherein the human V H, D, and J H gene segments are operably linked to an endogenous mouse heavy chain constant gene, and the human V H, D, and J H gene segments are capable of rearranging and forming a rearranged human/mouse chimeric heavy chain gene; (b) allowing the mouse to develop an immune response to the antigen of interest; and, (c) identifying a clonally selected lymphocyte of the mouse that expresses an antibody that specifically binds the antigen of interest, and obtaining from the lymphocyte or the antibody a nucleotide sequence that encodes a human heavy chain variable region of the antibody that specifically binds the antigen of interest. (a) immunizing a genetically modified mouse with an antigen of interest, wherein the mouse comprises (i) a replacement at the endogenous mouse κ immunoglobulin light chain variable region locus of all endogenous mouse κ immunoglobulin light chain variable region gene segments with a rearranged human Vκ1-39/J gene segment, a rearranged human Vκ3-20/J gene segment, or a combination thereof, wherein the rearranged human gene segment is operably linked to an endogenous mouse κ constant gene, wherein the mouse lacks an endogenous mouse κ immunoglobulin light chain variable region locus that is capable of rearranging and forming a gene that encodes a mouse κ variable region; and (ii) a replacement of 90-100% of unrearranged mouse V H gene segments with at least one unrearranged human V H gene segment, and a replacement of all of the endogenous mouse D and J H gene segments with at least one unrearranged human D gene segment and at least one unrearranged human J H gene segment, wherein the human V H, D, and J H gene segments are operably linked to an endogenous mouse heavy chain constant gene, and the human V H, D, and J H gene segments are capable of rearranging and forming a rearranged human/mouse chimeric heavy chain gene; (b) allowing the mouse to develop an immune response to the antigen of interest; and, (c) identifying a clonally selected lymphocyte of the mouse that expresses an antibody that specifically binds the antigen of interest, and obtaining from the lymphocyte or the antibody a nucleotide sequence that encodes a human heavy chain variable region of the antibody that specifically binds the antigen of interest.