Exons 2 - 6 were replaced by an FRT-flanked Pgk1 gene promoter-driven neomycin resistance gene (Pgk-neo) cassette. Western blot analysis revealed the complete absence of protein expression from myelin fractions of homozygous mutant brains at weaning, young adult and adult stages. Immunohistochemical analysis confirmed the absence of protein expression in the brains at 16 weeks of age. Expression of other tested major myelin components is similar to that in wild-type controls.