One possibility to explain these low levels, which however are similar to those achieved by others in MPS I cell lines, is that measuring ARSB activity may not be sensitive enough to detect low increases, since cultured MPS VI fibroblasts can uptake exogenous ARSB from the culture medium and this might result in variable degrees of baseline ARSB activity measured in control untreated fibroblasts. The gene discussed is ARSB; the disease is mucopolysaccharidosis type 6.