Recent reports questioned the possibility of cross-correction in the PNS of Twitcher mice8 or in the CNS of metachromatic leukodystrophy.67 In both articles, the authors investigated whether WT macrophages expressing natural levels of enzyme could support cross-correction; they failed to detect either GALC or arylsulfatase A in Schwann cells or oligodendrocytes when normal WT macrophages were present. This evidence concerns the gene GALC and metachromatic leukodystrophy.