Unexpectedly, the phenotypes of mice with an Fmrp I304N point mutation, which causes a clinically severe FXS in humans that includes profound intellectual disability and excessive macroorchidism (De Boulle et al., 1993), resemble those of Fmr1 KO mice; for example, in their degree of macroorchidism, in their behavioral responses and in their synaptic electrophysiological measurements (Zang et al., 2009). Here, FMR1 is linked to fragile X syndrome.