Enhanced immunoglobulin diversity

Isolate primary B cells, immortalized B cells, or hybridomas derived from a genetically modified mouse,

• wherein the genome of the genetically modified mouse comprises a first immunoglobulin heavy chain allele in which some or all constant domain (CH) exons of the first immunoglobulin heavy chain are flanked by oppositely oriented site-specific recognition sequences and a second immunoglobulin heavy chain allele in which a constant domain (CH) exon of the second immunoglobulin heavy chain allele is in antisense orientation with respect to transcriptional direction and is flanked by oppositely oriented site-specific recognition sequences;
• wherein the first immunoglobulin heavy chain allele is capable of expressing functional immunoglobulin heavy chain and the second immunoglobulin heavy chain allele can undergo productive VDJ rearrangement but is deficient in expression of a functional immunoglobulin heavy chain;
• and
• wherein the deficiency in expression of the second allele can be corrected by site-specific recombination to allow for production of a functional immunoglobulin heavy chain that has not been subjected to selection by tolerance mechanisms.