To evaluate the locus specificity of the HTT TP-PCR MCA assay, 6 samples with expansion mutations at the FMR1 CGG repeat locus responsible for fragile X syndrome (FXS), or the DMPK CTG repeat locus responsible for myotonic dystrophy type 1 (DM1), were tested. The gene discussed is FMR1; the disease is myotonic dystrophy type 1.