A method for providing a human light chain variable domain, comprising: immunizing a mouse with an antigen, wherein the genome of the mouse comprises: (a) an insertion of one or more human V L gene segments and one or more human J L gene segments upstream of a mouse immunoglobulin light chain constant region gene, wherein the one or more human V L gene segments and one or more human J L gene segments are operably linked to the mouse immunoglobulin light chain constant region gene; (b) an insertion of one or more human V L gene segments and one or more human J L gene segments upstream of a mouse immunoglobulin heavy chain constant region gene, wherein the one or more human V L gene segments and one or more human J L gene segments are operably linked to the mouse immunoglobulin heavy chain constant region gene; and (c) an ectopic nucleic acid sequence that encodes a mouse ADAM6a protein or a functional fragment thereof and an ectopic nucleic acid sequence that encodes a mouse ADAM6b protein or a functional fragment thereof, wherein the mouse ADAM6a and ADAM6b proteins or functional fragments thereof are expressed from the ectopic nucleic acid sequences; and wherein the mouse expresses an antibody that specifically binds the antigen, comprising (i) a polypeptide that comprises a human light chain variable domain linked to a mouse heavy chain constant domain, and (ii) a polypeptide that comprises a human light chain variable domain linked to a mouse light chain constant domain, isolating at least one cell from the mouse that produces the antibody that specifically binds the antigen, and providing a human light chain variable domain of the antibody. immunizing a mouse with an antigen, wherein the genome of the mouse comprises: (a) an insertion of one or more human V L gene segments and one or more human J L gene segments upstream of a mouse immunoglobulin light chain constant region gene, wherein the one or more human V L gene segments and one or more human J L gene segments are operably linked to the mouse immunoglobulin light chain constant region gene; (b) an insertion of one or more human V L gene segments and one or more human J L gene segments upstream of a mouse immunoglobulin heavy chain constant region gene, wherein the one or more human V L gene segments and one or more human J L gene segments are operably linked to the mouse immunoglobulin heavy chain constant region gene; and (c) an ectopic nucleic acid sequence that encodes a mouse ADAM6a protein or a functional fragment thereof and an ectopic nucleic acid sequence that encodes a mouse ADAM6b protein or a functional fragment thereof, wherein the mouse ADAM6a and ADAM6b proteins or functional fragments thereof are expressed from the ectopic nucleic acid sequences; and wherein the mouse expresses an antibody that specifically binds the antigen, comprising (i) a polypeptide that comprises a human light chain variable domain linked to a mouse heavy chain constant domain, and (ii) a polypeptide that comprises a human light chain variable domain linked to a mouse light chain constant domain, (a) an insertion of one or more human V L gene segments and one or more human J L gene segments upstream of a mouse immunoglobulin light chain constant region gene, wherein the one or more human V L gene segments and one or more human J L gene segments are operably linked to the mouse immunoglobulin light chain constant region gene; (b) an insertion of one or more human V L gene segments and one or more human J L gene segments upstream of a mouse immunoglobulin heavy chain constant region gene, wherein the one or more human V L gene segments and one or more human J L gene segments are operably linked to the mouse immunoglobulin heavy chain constant region gene; and (c) an ectopic nucleic acid sequence that encodes a mouse ADAM6a protein or a functional fragment thereof and an ectopic nucleic acid sequence that encodes a mouse ADAM6b protein or a functional fragment thereof, wherein the mouse ADAM6a and ADAM6b proteins or functional fragments thereof are expressed from the ectopic nucleic acid sequences; isolating at least one cell from the mouse that produces the antibody that specifically binds the antigen, and providing a human light chain variable domain of the antibody.