The targeting vector is designed with (from 5' to 3') a CMV-IE enhancer/chicken beta-actin/rabbit beta-globin hybrid promoter (CAG), a loxP-flanked STOP cassette, a flpo recombinase gene (a codon-optimized flpe gene modified for translation in mammalian cells and higher recombination efficiency in cells and in mice), and a Neo-polyA cassette. This entire construct was inserted between exons 1 and 2 of the Gt(ROSA)26Sor locus. (J:235123)