A loxP site was inserted upstream of exon 4 and a lox2272 site was inserted downstream of exon 4 to allow for conditional removal of the wild-type exon 4 and expression of a mutagenized cDNA for exons 4 and 5 with nucleotide substitutions that result in the amino acid substitutions of alanine for 11 serine or threonine. An FRT-flanked neomycin resistance cassette was inserted downstream of the lox2272 site in intron 4. The selection cassette was removed. (J:257168)