ARSA and metachromatic leukodystrophy: Atidarsagene autotemcel (arsa-cel) is a gene therapy medicinal product based on autologous haematopoietic stem and progenitor cells (HSPCs) transduced ex vivo with a lentiviral vector encoding the ARSA cDNA driving supranormal ARSA expression in HSPCs and their progeny, which has previously shown preliminary evidence of safety and efficacy in nine patients with early-onset MLD.16, 17 In this Article, we present results from 29 patients with early-onset MLD (16 with late-infantile MLD, 13 with early-juvenile MLD) treated with arsa-cel, with maximum follow-up of 7·5 years.