ARSB and mucopolysaccharidosis type 6: Indeed, no improvement in bone abnormalities was observed in adult mice (p30) treated with high doses of vector or frequent regiment of ERT.15 Also, no response was observed in mice following neonatal gene transfer.15 In MPS VI transgenic mice treated at birth, we found that serum ARSB peaks to supraphysiological levels and then rapidly drops to below normal levels few weeks after injection (unpublished data), because of vector dilution due to hepatocytes proliferation.56,57 This may explain the absence of bone responses after neonatal gene transfer in MPS VI mice.