The targeting vector is designed with a loxP site downstream of the endogenous exon 1 of the gene followed by exons 2-8. A transcriptional STOP sequence (3 repeats of SV40 late poly A sequence) followed by an FRT-flanked neomycin selection cassette and a second loxP site were inserted downstream of the exon 2-8 sequence of the targeting construct. A missense mutation (TCT > TTT) was created in the endogenous exon 2, resulting in a serine to phenylalanine mutation at position 34 (S34F).