DARS2 and spastic ataxia: Unlike most mutations in mitochondrial translation proteins that cause predominantly neuronal defects, mutations in DARS2, encoding the mitochondrial aspartyl-tRNA synthetase, cause white matter disease-leukoencephalopathy with brainstem and spinal cord involvement, and lactate elevation (LBSL) characterized by progressive spastic ataxia and multiple long-tract involvements.