For instance, a correlation between residual ARSA enzyme activity and the severity of clinical presentation was reported in MLD patients [61], and longitudinal assessments showed that there were decreases in dried blood spot (DBS) concentrations of psychosine, a substrate of the GALC enzyme, associated with natural disease progression and treatment with HSCT in Krabbe disease patients [62, 63]. This evidence concerns the gene GALC and Krabbe disease.