ARSA and metachromatic leukodystrophy: Next, the FVV:PGK-ARSA transduced mMSCs cell-culture medium, or that from FVV:PGK-GFP (control medium), was incubated with normal (functional ARSA) or MLD patients' (ARSA deficient) fibroblasts that had been preloaded with fluorescently labelled substrate (BODIPY-sulphatide).