Mice that make VL binding proteins

A targeting vector comprising from 5′ to 3′:

• (a) a first homology arm,
• (b) a nucleic acid sequence comprising one or more human immunoglobulin (Ig) light chain variable kappa (Vκ) gene segments and one or more human Ig light chain joining kappa (Jκ) gene segments,
• wherein the one or more human Ig Vκ gene segments and the one or more human Ig Jκ gene segments are capable of rearranging in a B cell to form a rearranged Vκ/Jκ sequence, and
• (c) a second homology arm,
• wherein
• the targeting vector is capable of (i) replacing a plurality of mouse Ig heavy chain V gene segments, all mouse Ig heavy chain D gene segments, and all mouse Ig heavy chain J gene segments of a mouse Ig heavy chain gene with the one or more human Ig Vκ gene segments and the one or more human Ig Jκ gene segments and (ii) operably linking the one or more human Ig Vκ gene segments and the one or more human Ig Jκ gene segments to an Igu gene of the mouse Ig heavy chain gene.