Pt053, a boy with complex I deficiency and seizures, diarrhea, arrhythmia, regression, respiratory failure, liver dysfunction, and hearing loss, harbored the hemizygous de novo mutation c.806delG (p.G269fs, rs61750241) in MECP2 (NM_004992) (S15 Fig), a gene reported to cause Rett syndrome (MIM 312750). The gene discussed is MECP2; the disease is Rett syndrome.