FMR1 and fragile X syndrome: For known monogenic NDD syndromes, genetic mouse models such as the Fragile X mental retardation 1 knockout (Fmr1-KO) mouse for Fragile X syndrome (FXS; Bakker et al., 1994) or tuberous sclerosis protein 1/2 (TSC 1/2) models for tuberous sclerosis (Meikle et al., 2007; Ehninger et al., 2008) have enabled the functional study of these genes in the intact brain.