CD34 and metachromatic leukodystrophy: Three children with ARSA deficiency and mutations associated with early-onset MLD were included in a phase 1/2 trial, carried out in a partnership between Orchard Therapeutics and San Raffaele-Telethon Institute for Gene Therapy, and treated at the pre-symptomatic stage with autologous CD34-positive HSPCs transduced ex vivo with a lentivector carrying the ARSA gene (OTL-200; Biffi et al., 2013).