A method of producing an antibody in a genetically modified mouse, the method comprising the steps of: (a) immunizing the genetically modified mouse with an antigen of interest, wherein the genetically modified mouse has a germline genome comprising: a first engineered endogenous immunoglobulin κ light chain locus comprising: (i) one or more human Vλ gene segments, (ii) one or more human Jλ gene segments, and (iii) one mouse Cλ gene, wherein the one or more human Vλ gene segments of (i) and the one or more human Jλ gene segments of (ii) are operably linked to the one mouse Cλ gene of (iii); wherein the one mouse Cλ gene (iii) is in the place of a mouse Cκ gene at the first engineered endogenous immunoglobulin κ light chain locus; wherein the one or more human Vλ gene segments of (i) and the one or more human Jλ gene segments of (ii) are in place of one or more endogenous mouse Vκ gene segments and one or more endogenous mouse Jκ gene segments; wherein the genetically modified mouse lacks a mouse Cκ gene at the first engineered endogenous immunoglobulin k light chain locus; (b) maintaining the genetically modified mouse under conditions sufficient for the genetically modified mouse to produce an immune response to the antigen of interest; and (c) recovering from the genetically modified mouse: (i) an antibody that binds the antigen of interest, wherein the antibody comprises a light chain comprising a human λ variable domain and a mouse λ constant domain, (ii) a nucleotide that encodes a human light chain variable domain or human heavy chain variable domain, a light chain, or a heavy chain of an antibody that binds the antigen of interest, wherein the antibody that binds the antigen of interest comprises a light chain comprising a human λ variable domain and a mouse λ constant domain, or (iii) a cell that expresses an antibody that binds the antigen of interest, wherein the antibody comprises a light chain comprising a human λ variable domain and a mouse λ constant domain. (a) immunizing the genetically modified mouse with an antigen of interest, wherein the genetically modified mouse has a germline genome comprising: a first engineered endogenous immunoglobulin κ light chain locus comprising: (i) one or more human Vλ gene segments, (ii) one or more human Jλ gene segments, and (iii) one mouse Cλ gene, wherein the one or more human Vλ gene segments of (i) and the one or more human Jλ gene segments of (ii) are operably linked to the one mouse Cλ gene of (iii); wherein the one mouse Cλ gene (iii) is in the place of a mouse Cκ gene at the first engineered endogenous immunoglobulin κ light chain locus; wherein the one or more human Vλ gene segments of (i) and the one or more human Jλ gene segments of (ii) are in place of one or more endogenous mouse Vκ gene segments and one or more endogenous mouse Jκ gene segments; wherein the genetically modified mouse lacks a mouse Cκ gene at the first engineered endogenous immunoglobulin k light chain locus; a first engineered endogenous immunoglobulin κ light chain locus comprising: (i) one or more human Vλ gene segments, (ii) one or more human Jλ gene segments, and (iii) one mouse Cλ gene, (i) one or more human Vλ gene segments, (ii) one or more human Jλ gene segments, and (iii) one mouse Cλ gene, wherein the one or more human Vλ gene segments of (i) and the one or more human Jλ gene segments of (ii) are operably linked to the one mouse Cλ gene of (iii); wherein the one mouse Cλ gene (iii) is in the place of a mouse Cκ gene at the first engineered endogenous immunoglobulin κ light chain locus; wherein the one or more human Vλ gene segments of (i) and the one or more human Jλ gene segments of (ii) are in place of one or more endogenous mouse Vκ gene segments and one or more endogenous mouse Jκ gene segments; wherein the genetically modified mouse lacks a mouse Cκ gene at the first engineered endogenous immunoglobulin k light chain locus; (b) maintaining the genetically modified mouse under conditions sufficient for the genetically modified mouse to produce an immune response to the antigen of interest; and (c) recovering from the genetically modified mouse: (i) an antibody that binds the antigen of interest, wherein the antibody comprises a light chain comprising a human λ variable domain and a mouse λ constant domain, (ii) a nucleotide that encodes a human light chain variable domain or human heavy chain variable domain, a light chain, or a heavy chain of an antibody that binds the antigen of interest, wherein the antibody that binds the antigen of interest comprises a light chain comprising a human λ variable domain and a mouse λ constant domain, or (iii) a cell that expresses an antibody that binds the antigen of interest, wherein the antibody comprises a light chain comprising a human λ variable domain and a mouse λ constant domain. (i) an antibody that binds the antigen of interest, wherein the antibody comprises a light chain comprising a human λ variable domain and a mouse λ constant domain, (ii) a nucleotide that encodes a human light chain variable domain or human heavy chain variable domain, a light chain, or a heavy chain of an antibody that binds the antigen of interest, wherein the antibody that binds the antigen of interest comprises a light chain comprising a human λ variable domain and a mouse λ constant domain, or (iii) a cell that expresses an antibody that binds the antigen of interest, wherein the antibody comprises a light chain comprising a human λ variable domain and a mouse λ constant domain.