Following on from work in MLD (metachromatic leukodystrophy), MPSI, MPSIIIA and MPSIIIB, where supraphysiological enzyme levels achieved after HSCGT have been shown to correct neurological disease manifestations in mouse models [52,53,54,55,56], further improvements to vector design, to allow trafficking of the delivered enzyme across the blood–brain barrier, have been shown to completely normalise brain pathology and behaviour in MPSII mice [57]. The gene discussed is IDUA; the disease is metachromatic leukodystrophy.