A floxed neo cassette was inserted upstream of exon 46 and exon 47 was replaced with a modified human exon 47 containing 114 CAG repeats interrupted by 4 CAA units in multiple places. A splice site mutation was inserted flanking the repeat region preventing production of the MPc and MPII isoforms. Cre meditated recombination removed the floxed neo cassette. Quantitative RT-PCR detected similar levels of expression of transcript in the cerebellum of homozygous mutant and wild-type mice. Sequencing of cDNAs from cerebellar extracts confirmed the absence of the MPc and MPII isoforms in homozygous mice.