Using homologous recombination in ES cells, the targeting vector inserted an Frt flanked neomycin cassette downstream of Xbp1 exon 5, two loxP sites flanking the exons 4 and 5 together with the neo-cassette, and a modified version of Xbp1 exon 4 (E426) along with exon 5 was positioned downstream of the second loxP site. The exon E426 is devoid of the intronic sequence of 26 nucleotides. (J:314673)