FMR1 and fragile X syndrome: The fmr1-KO mouse shows no detectable levels of fmr1 mRNA or FMR protein and displays many of the physical and neurobiological characteristics expected of humans with FXS, for example, macroorchidism, prevalence of seizure disorders, reduced cerebellar volume and abnormalities in dendritic spines.15, 16, 17, 18, 19 However, the fmr1-tm1Cgr mouse model is not a complete molecular null model,20 which may contribute to discrepancies in findings.