However, even in these Mendelian disorders, there is incomplete penetrance of autism and variable phenotypic presentations; for example, although fragile X syndrome is considered a syndromic form of autism, only around 30% of fragile X syndrome patients (who carry mutations in the FMR1 gene) are diagnosed with autism (Fatemi and Folsom, 2011). Here, FMR1 is linked to fragile X syndrome.