The targeting construct contains loxP sites flanking the mouse Ins2 gene. An Frt-flanked Neo cassette was inserted after the Ins2 exon 3 before the loxP site. The construct was injected into ES cells to modify the endogenous locus through homologous recombination. Mice expressing this allele were generated and subsequently crossed to 129S4/SvJaeSor-Gt(ROSA)26Sortm1(FLP)Dym/J mice to delete the Neo cassette.