For example, metachromatic leukodystrophy (MLD) is a lysosomal storage syndrome that is promoted by deficiency of the enzyme arylsulfatase (ARSA; EC 3.1.6.8) and, in some cases, because of defects in the activity of the ARSA cofactor saposin B (Cesani et al., 2016), both of which are required for sulfatide turnover (Figure 1). Here, ARSA is linked to metachromatic leukodystrophy.