FMR1 and fragile X syndrome: There is reduced or delayed myelination in FXS in many brain areas (Pacey et al., 2013; Phan et al., 2020) and one of the targets of FMRP is myelin basic protein (Jeon et al., 2017), suggesting that deficits seen in FXS may be caused at least in part by alterations to myelination.