Exons 1 to 2 and approx. 1.6 kb of sequence upstream of exon 1 (promoter region) were flanked by loxP sites (size of loxP-flanked region: 3.7 kb). The conditional allele was generated after Flp-mediated recombination. Cre-mediated recombination removed the floxed segment. (J:147112)