Mitochondrial diseases, inborn errors of metabolism, neurodevelopmental disorders, and glycogen storage diseases (GSD), such as those caused by NFU1, Cobalamin C (CblC) disease, MECP2 variants, or GBE1, respectively, should be covered by the genetic techniques used in childhood-onset cases of PAH [28,29,30,31,32]. Here, MECP2 is linked to mitochondrial disease.