FMR1 and fragile X syndrome: This may have implications for (i) patient stratification in clinical trials and the outcome measures used in stratified subgroups; (ii) design of treatment strategies aiming to re-activate FMR1 in FXS, which may result in production of harmful FM mRNA by a small proportion of cells; and (iii) preclinical trials targeting downstream pathways to both FM RNA toxicity as well as FMRP deficiency, as opposed to widely used, FXS KO models, where FM RNA toxicity cannot be studied.