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An expanded tract of 154 CAG repeats was engineered and knocked into exon 7 by replacing the PPPQQ codons with 154 glutamine (Q) codons. Additionally, a loxP site flanked neomycin resistance gene cassette was inserted into intron 6 and subsequently deleted in ES cells via Cre-mediated recombination. While equivalent transcript levels were identified in both mutant and wild-type mice by RT-PCR, reduced levels of mutant protein were observed in brain tissue. The authors attributed this apparent reduction in protein level to an increased difficulty of extraction or solubilization due to the mutant protein accumulating into nuclear aggregates as mice age. (J:77225)