Non-human animals (and/or non-human cells) and methods of using and making the same are provided, which non-human animals (and/or non-human cells) have a genome comprising human antibody-encoding sequences (i.e., immunoglobulin genes). Non-human animals described herein express antibodies that contain human Igλ light chains, in whole or in part. In particular, non-human animals provided herein are, in some embodiments, characterized by expression of antibodies that contain human Igλ light chains, in whole or in part, that are encoded by human Igλ light chain-encoding sequences inserted into an endogenous Igλ light chain locus of said non-human animals. Methods for producing antibodies from non-human animals are also provided.