Genetically modified mice are provided that express human λ variable (hVλ,) sequences, including mice that express hVλ sequences from an endogenous mouse λ light chain locus, mice that express hVλ, sequences from an endogenous mouse κ light chain locus, and mice that express hVλ, sequences from a transgene or an episome wherein the hVλ, sequence is linked to a mouse constant sequence. Mice are provided that are a source of somatically mutated human λ variable sequences useful for making antigen-binding proteins. Compositions and methods for making antigen-binding proteins that comprise human λ variable sequences, including human antibodies, are provided.