The targeting construct consisting of a loxP site, exons 4-7 with a polyA site, FRT-flanked neomycin resistance cassette, second loxP site and exons 4-7 containing an arginine to glutamine substitution at residue 183 (p.R183Q) and a polyA site replaced exon 4. The neomycin selection cassette was removed via flp-mediated recombination. (J:339551)