FMR1 and fragile X syndrome: Fragile X syndrome (FXS), the most common form of inherited intellectual disability [1], results from a trinucleotide repeat expansion in the 5′-untranslated region of the Fragile X Mental Retardation 1 (FMR1) gene [2] which causes the transcriptional silencing and loss of its encoded protein, the fragile X mental retardation protein (FMRP) [3,4].