FMR1 and fragile X syndrome: The fragile X syndrome (FXS), the most common form of inherited intellectual disability (ID), is typically caused by an expansion of the CGG trinucleotide repeat in the non-coding 5′ untranslated region of the FMR1 gene, leading to hypermethylation of the genes, transcriptional silencing, and thus loss of the FMRP protein.