A 250 kb YAC containing the intact human locus with 15 CAG trinucleotide repeats in exon 10 was modified by homologous recombination to expand the number of CAG repeats to 76. Two copies of the transgene inserted into this line with the number of CAG repeats for both copies truncated to 67 as determined by sequencing (a second founder line created with this construct, 72.1, had insertion of one transgene into the X chromosome with 72 CAG repeats). Transgene expression was detected in the cerebellum at endogenous levels and was also detected in the cerebral cortex, heart, and skeletal muscle.