DARS1 and hypomyelination with brain stem and spinal cord involvement and leg spasticity: 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.