Two loxP sites were introduced to the intron and 5' untranslated region flanking exon 1, and a third loxP site was introduced with a PGK-TK/PGK-neo cassette directly downstream exon 1 via homologous recombination. Exon 1 and the PGK-TK/PGK-neo cassette were removed by transient expression of cre recombinase in correctly targeted ES cells resulting in the deletion of 2.1 kb of DNA encoding amino acids 1-518 which include the whole motor region and half of the alpha-helical coiled coil domains. Northern blot analysis detected a truncated transcript encoding the tail end of the gene in homozygous mutant animals. Truncated protein product was detected in homozygous mutants by Western blot analysis using antibodies recognizing the tail end of the protein. Immunoprecipitation of brain lysates from homozygous mutants demonstrated the ability of the truncated protein to specifically bind Kif3a and not Kif3b.