While H1 ESCs (male) carrying a typical FMR1 allele with 30 CGG repeats (Fig. 1A) showed FMR1 mRNA expression that was 120% of GUSB expression, FMR1 mRNA levels in FXS ESCs were only 0.05% of GUSB (Fig. 1B). This evidence concerns the gene GUSB and fragile X syndrome.