Mice that make VL binding proteins

A mouse whose germline genome comprises a modified endogenous mouse immunoglobulin heavy chain locus comprising a replacement of all functional endogenous mouse immunoglobulin heavy chain variable (VH) gene segments, all functional endogenous mouse immunoglobulin heavy chain diversity (DH) gene segments and all functional endogenous mouse immunoglobulin heavy chain joining (JH) gene segments at the endogenous mouse immunoglobulin heavy chain locus with a nucleotide sequence that comprises a plurality of contiguous unrearranged functional human immunoglobulin light chain variable Vκ (hVκ) gene segments and all five unrearranged functional human immunoglobulin light chain joining gene segments (hJκ1-hJκ5),

• wherein the plurality of unrearranged functional hVκ gene segments and the five unrearranged functional human immunoglobulin light chain joining gene segments (hJκ1-hJκ5) are operably linked to an intact endogenous mouse immunoglobulin heavy chain constant region at the endogenous mouse immunoglobulin heavy chain locus,
• wherein the plurality of unrearranged functional hVκ gene segments and the five unrearranged functional human immunoglobulin light chain joining gene segments (hJκ1-hJκ5) rearrange in a B cell during B cell development to form a rearranged human immunoglobulin light chain variable region Vκ/Jκ nucleotide sequence operably linked to the endogenous mouse immunoglobulin heavy chain constant region at the endogenous mouse immunoglobulin heavy chain locus, and
• wherein the mouse comprises a CD19+ B cell comprising the rearranged human immunoglobulin light chain variable region Vκ/Jκ nucleotide sequence operably linked to the endogenous mouse immunoglobulin heavy chain constant region at the endogenous mouse immunoglobulin heavy chain locus.