The underlying cause of HBSL are missense mutations of the aspartyl-tRNA synthetase (AspRS) gene DARS1. AspRS belongs to a group of enzymes termed aminoacyl-tRNA synthetases (ARSs) that catalyze an aminoacylation reaction in which transfer ribonucleic acids (tRNAs) are linked to their cognate amino acids. This evidence concerns the gene DARS2 and hypomyelination with brain stem and spinal cord involvement and leg spasticity.