In stark contrast to the published data for BSC [4,5,6], the published clinical data for arsa-cel show that with up to approximately 11 years of follow-up in 39 patients, arsa-cel provides meaningful clinical benefit to patients with early-onset MLD treated in both the pre-symptomatic and early symptomatic stages of the disease [3,12]. The gene discussed is ARSA; the disease is metachromatic leukodystrophy.