F-box and leucine-rich repeat protein 4 (FBXL4) mitochondrial disease is an autosomal recessive disorder that manifests with a highly variable breadth and severity of multisystem features, including encephalomyopathy, global developmental delay, epilepsy, dysmorphic features, skeletal abnormalities, hypertrophic cardiomyopathy, arrhythmias, liver dysfunction, visual impairment, hearing loss, bone marrow deficiencies, renal tubular acidosis requiring bicarbonate supplementation, and pronounced lactic acidemia (1–5). This evidence concerns the gene FBXL4 and mitochondrial DNA depletion syndrome, encephalomyopathic form with methylmalonic aciduria.