DARS1 and hypomyelination with brain stem and spinal cord involvement and leg spasticity: Hypomyelination with brain stem and spinal cord involvement and leg spasticity (HBSL, OMIM: 615281) is a leukodystrophy disease caused by a recessive missense mutation of the DARS1 gene encoding cytoplasmic aspartyl-tRNA synthetase (AspRS).