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Objective-To determine how neuroanatomic variation in children and adolescents with fragile X syndrome is linked to reduced levels of the fragile X mental retardation-1 protein and to aberrant cognition and behavior.Methods-This study included 84 children and adolescents with the fragile X full mutation and 72 typically developing control subjects matched for age and sex. Brain morphology was assessed with volumetric, voxel-based, and surface-based modeling approaches. Intelligence quotient was evaluated with standard cognitive testing, whereas abnormal behaviors were measured with the Autism Behavior Checklist and the Aberrant Behavior Checklist.Results-Significantly increased size of the caudate nucleus and decreased size of the posterior cerebellar vermis, amygdala, and superior temporal gyrus were present in the fragile X group. Subjects with fragile X also demonstrated an abnormal profile of cortical lobe volumes. A receiver operating characteristic analysis identified the combination of a large caudate with small posterior cerebellar vermis, amygdala, and superior temporal gyrus as distinguishing children with fragile X from control subjects with a high level of sensitivity and specificity. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript cerebellar vermis were associated with lower fragile X mental retardation protein levels and more pronounced cognitive deficits and aberrant behaviors.Interpretation-Abnormal development of specific brain regions characterizes a neuroanatomic phenotype associated with fragile X syndrome and may mediate the effects of FMR1 gene mutations on the cognitive and behavioral features of the disorder. Fragile X syndrome provides a model for elucidating critical linkages among gene, brain, and cognition in children with serious neurodevelopmental disorders.Cognitive dysfunction and aberrant behaviors are the hallmarks of childhood neurodevelopmental disorders. Though current diagnostic constructs for these disorders are useful for professional communication and obtaining therapeutic services, the current taxonomic categories are likely to be too heterogeneous to support scientific investigation. 1 More homogeneous models are needed to overcome the problem of heterogeneity in our current taxonomy of neurodevelopmental disorders, so that genetic and neural mechanisms underlying the development and course of maladaptive cognition and behavior can be determined. 2Fragile X syndrome (FraX) is caused by a mutation in the FMR1 gene on chromosome Xq27.3; it is the most common known cause of inherited neurodevelopmental disability. As a causatively homogeneous disorder where affected children share a common genetic risk factor, FraX is a valuable model from which to learn about pathways leading from specific gene mutation to aberrant brain development and cognitive-behavioral symptoms. 2 Accordingly, the aim of this study was to determine the associations among levels of FMRP, neuroanatomic variation, and aberrant cognition and behavior in FraX using multi...