Metachromatic leukodystrophy (MLD) is a lysosomal storage disease caused by deficiency of the enzyme arylsulfatase A (ARSA). MLD is characterized by progressive demyelination and neurological deficits. Treatment of MLD is still a challenge due to the fact that the blood-brain barrier is a major obstacle for most therapeutic substances. In this issue of the JCI, Biffi et al. report that genetically modified hematopoietic precursor cells transduced to overexpress ARSA and transplanted into mice with a targeted disruption of the murine Arsa gene (Arsa(-/-) mice) migrated into the CNS and cross-corrected brain ARSA deficiency (see the related article beginning on page 3070). Microglia served as a cellular vehicle to effectively deliver the enzyme to other brain cells while hepatocytes overexpressing ARSA increased plasma ARSA levels but failed to deliver ARSA into the CNS.