Arylsulfatase A (ARSA) is an enzyme responsible for breaking down sulfatides within lysosomes. Genetic variants in ARSA result in a reduction of lysosomal ARSA enzymatic activity and cause Metachromatic Leukodystrophy (MLD). MLD is characterized by an accumulation of sulfatides leading to neuronal degradation resulting in progressive cognitive and motor decline. ETV:ARSA is a recombinant ARSA enzyme engineered to cross the blood-brain barrier and to replace ARSA to treat neuropathic and systemic manifestations of the disease.