The ASO–CCG-induced increase in FMR1 pre-mRNA and mRNA is significantly milder in FXTAS-derived cells with RNase H1 deficiency, which generally reduces the negative effect of R-loops on transcription (Fig. 3a, b and Supplementary Fig. S3a). The gene discussed is RNASEH1; the disease is fragile X-associated tremor/ataxia syndrome.