The targeting vector was designed to insert a cassette containing synthetic LacO sequences flanking the Htt transcriptional start site followed by a chimeric mouse/human Htt sequence with human exon 1 into the mouse Htt locus on chromosome 5. The human sequence encodes the N terminal sequence including an expanded CAG repeat containing 140Q and the adjacent sequence encoding the human proline-rich region. A loxP-flanked PGK-neomycin cassette was inserted in reverse orientation 1.3 kb upstream of transcription initiation site (J:333052)