Several autism-related neurodevelopmental disorders, such as Fragile X, Rett, Angelman syndromes (AS), and tuberous sclerosis are caused by a highly penetrable mutation of a single gene, e.g., FMR1 in Fragile X (Verkerk et al., 1991; Gedeon et al., 1992), MECP2 in Rett (Amir et al., 1999), UBE3A in AS (Kishino et al., 1997), and TSC1/2 in tuberous sclerosis complex (TSC; Povey et al., 1994). Here, TSC1 is linked to tuberous sclerosis.