This is a critical feature to consider when assessing the mechanisms behind FXS pathology, but also when choosing research tools, because the CGG-repeat expansion at the FMR1 locus, and its ensuing developmentally-regulated disappearance of FMRP, is unique to humans and is not recapitulated in Fmr1 knock-out (KO) mice (Eiges et al., 2007; Telias and Ben-Yosef, 2014). The gene discussed is FMR1; the disease is fragile X syndrome.