FMR1 and fragile X syndrome: FXS, the most common monogenic cause of ASD and ID (Richter, Bassell, & Klann, 2015), is caused by expansion of trinucleotide cytosine‐cytosine‐guanine (CGG) repeats in the FMR1 gene leading to transcriptional silencing of the FMR1 locus and loss of FMRP protein function.