A genotype–phenotype correlation has been proposed based on the predicted functionality of disease‐causing ARSA variants: variants resulting in very low or absent residual ASA activity (0‐alleles) are linked to early‐onset MLD phenotypes, while variants resulting in higher activity (R‐alleles) are associated with late‐onset phenotypes. The gene discussed is ARSA; the disease is metachromatic leukodystrophy.