XYLT1 and fragile X syndrome: Archetype of expansions shutting down gene expression are the GGC, CGG, and GAA expanded repeats located within the promoter, 5′UTR and first intron of the XYLT1, FMR1, and frataxin genes and associated with Baratela-Scott syndrome (BBS), fragile X syndrome (FXS), and Friedreich ataxia (FA), respectively, (Fu et al., 1991; Oberlé et al., 1991; Verkerk et al., 1991; Kremer et al., 1991; Campuzano et al., 1996; laCroix et al., 2019; Table 1).