A loxP site was inserted upstream of exon 4 and a loxP site and frt flanked neo cassette were inserted downstream of exon 4 via homologous recombination. Cre mediated recombination deleted exons 4 and 5, which encode most of the MH2 domain, leaving a single loxP site and the frt flanked neo cassette. (J:128934, J:150213)