FMR1 and Rett syndrome: Given that miRNA gene silencing occurs in many tissue types including in the developing brain [213], it is intriguing that four proteins critical for miRNA biogenesis [214] are encoded by genes associated with Mendelian disorders where ASD and overlapping neurobehavioral phenotypes are highly prevalent: DRCG8 (included within the deleted region in chromosome 22q11.2) [215], MECP2 (Rett syndrome) [216], FOXG1 [217] and FMR1 (Fragile X) [218, 219].