FMR1 and fragile X syndrome: Although the genetic cause of FXS, a CGG trinucleotide repeat expansion in the 5′ untranslated region (UTR) of the Fragile X Messenger Riboprotein gene 1 (FMR1) leading to loss of the Fragile X Messenger Riboprotein (FMRP), was discovered more than two decades ago, many aspects of the disorder are still incompletely understood3.