A transgenic animal is provided. In certain embodiments, the transgenic animal comprises a genome comprising: an immunoglobulin light chain locus comprising: a) a functional immunoglobulin light chain gene comprising a transcribed variable region encoding: i. light chain CDR1, CDR2 and CDR3 regions that are composed of 2 to 5 different amino acids; and ii. a light chain framework; and, operably linked to the functional immunoglobulin light chain gene: b) a plurality of pseudogene light chain variable regions each encoding: i. light chain CDR1, CDR2 and CDR3 regions that are composed of the same 2 to 5 different amino acids as the CDRs of the functional gene; and ii. a light chain framework that is identical in amino acid sequence to the light chain framework of the transcribed variable region.