One widely studied specific sex chromosome alteration that has been associated with neuropsychiatric conditions is Fragile X syndrome (FXS), which is caused by the expansion of a trinucleotide repeat in the Fragile X Mental Retardation 1 (FMR1) gene on the X chromosome. In addition to being the most common inherited cause for ASD and intellectual disability, deletion of the FMR1 gene is also associated with a broad range of neuropsychiatric outcomes in both youth and adults ranging from anxiety disorders to substance abuse. 53 Studies of female carriers of the Fragile X mutation have also informed sex differences in the influence of these mutations. 54 Premutation carriers of the FMR1 gene have a lesser number of repeats (55‐200 CGG repeats) than those who manifest FXS (>200 CGG repeats), and increased prevalence of anxiety, depression, ASD, ADHD, intellectual and learning disabilities, substance use problems, and personality disorders have been reported. 53 , 55 In addition, a number of X chromosome genes and copy number variants (CNV) have been associated with intellectual disability, 56 developmental delay, 57 and schizophrenia, 58 but the risk of incurring a mutation associated with intellectual disability on the X chromosome is the same for males and females.
