A 15-year-old girl born to consanguineous parents had normal psychomotor development to age 12, then had episodes of proximal muscle weakness, myoglobinuria, lactic acidosis, and increased serum creatine kinase; a homozygous missense mutation was identified by whole exome sequencing in the initiation codon of the FDX1L (FDX2) gene (90). The gene discussed is FDX2; the disease is lactic acidosis.