For example, metachromatic leukodystrophy (MLD) is caused by the deficiency of lysosomal enzyme arylsulfatase A (ARSA), which leads to the accumulation of sphingolipid sulfatide in the oligodendrocytes, causing oligodendrocytal degeneration and progressive demyelination [140]. The gene discussed is ARSA; the disease is metachromatic leukodystrophy.