An FRT-flanked neomycin resistance cassette with a 3' loxP site was inserted upstream of exon 4. An additional loxP site was inserted downstream of exon 4. Flp-mediated recombination removed the selection cassette. Cre-mediated recombination removed exon 4. This mutation resembles the ~491+1G to A mutation observed in patients with congenital tufting enteropathy. (J:210901)