All of exons 17 through 21 and a portion of exon 22 were replaced by a cassette containing neo and hprt transgenes. The insertion is upstream of sequence encoding the DNA helicase domains 4 through 6 and the putative DNA-binding domain. The potential excision of the cassette generates a frameshift mutation via the splicing of exons 16 and 23. The targeting vector was designed such that it would not interfere with the Klc3 locus on the antisense strand.