Metachromatic leukodystrophy (MLD) arises from a lysosomal enzyme arylsulfatase A (ARSA) deficiency and, with a progressive accumulation of sulfatides in cells and myelin sheaths, leads to an increasing severe motor and cognitive disability (Kehrer, 2011, Gieselmann and Krageloh-Mann, 2010, Kehrer, 2021, Fumagalli, 2021). The gene discussed is ARSA; the disease is metachromatic leukodystrophy.