Exons 5 to 7 were flanked by loxP sites and an FRT-flanked selection cassette was inserted upstream of the second loxP site (downstream of exon 7). Downstream of the floxed region, exon 5 containing an arginine to serine substitution at amino acid 98 (p.R98S) was inserted. Prior to cre recombination, the wild-type gene is expressed. Cre-mediated recombination can be used to remove the floxed region which results in expression of the mutant protein. The selection cassette was removed via flp-mediated recombination. (J:271342)