The human BAC (RP11-33A21) was modified to include a transcriptional stop sequence (GFP followed by a floxed pgk-neo cassette and a polyA signal) before exon 1 and an expanded CTG/CAG track of 120 repeats in exon 2A of JPH3. RT-PCR confirmed that expression of the sense strand CUG repeat is silenced but expression of the antisense CAG repeat is retained.