Future studies should also examine: (i) GUS expression at both mRNA and protein levels in adults with FXSD including FXS, to rule out any associations with FMRP deficit; and (ii) to compare SDHA and EIF4A2 normalization strategy as well as the ratio between the two internal controls across FXSD including FXS. This evidence concerns the gene EIF4A2 and fragile X syndrome.