Non-human animals, cells, methods and compositions for making and using the same are provided, wherein the non-human animals and cells comprise an immunoglobulin heavy chain locus that includes unrearranged human immunoglobulin light chain gene segments and an immunoglobulin light chain locus that includes a single rearranged human light chain variable region nucleotide sequence. The unrearranged human light chain gene segments may be operably linked to a heavy chain constant region nucleotide sequence and the rearranged human immunoglobulin light chain variable region nucleotide sequence may be operably linked to a light chain constant region nucleotide sequence. Also provided are methods for obtaining nucleic acid sequences that encode immunoglobulin light chain variable domains capable of binding an antigen in the absence of a cognate variable domain, and expressing such nucleic acid sequences in a host cell, e.g., to generate a multispecific antigen-binding protein.