FMR1 and fragile X syndrome: The FMR1 gene is located on each X chromosome, in which the CGG triplet repeats a certain number of times; when the expansion of the nitrogenous bases exceeds 200 repetitions, methylation of the promoter site is observed, resulting in the silencing of the gene, in the total or partial absence of the FMRP protein for which it encodes, and thus in the manifestation of FXS.