ARSA and metachromatic leukodystrophy: For example, metachromatic leukodystrophy (MLD), one of the most common lysosomal storage diseases, is caused by a deficiency of arylsulphatase A (ARSA) causing storage of sulphatide in oligodendrocytes, Schwann cells, and neurons leading to progressive demyelination and, in its most common form, death by 5 years of age.17 Overexpression of ARSA in hematopoietic stem cells can reduce sulphatide storage caused by ARSA deficiency in the CNS of mice,18 despite efficacy being dependent on the recruitment of microglia across the blood-brain-barrier.