Exon 3 was replaced with one in which a C to G transversion (C387G) results in the amino acid substitution of tryptophan for cysteine at position 121 (C121W), mimicking a mutation found in some genetic epilepsy with febrile seizures plus (GEFS+) patients. A loxP was inserted upstream of the modified exon 3. A floxed neo cassette inserted downstream of the modified exon was removed by cre mediated recombination. The modified exon 3 remains floxed.