The diagnosis of USH type IV, rather than Sanfilippo disease, in the above described patients, was based on the fact that decreased ARSG activity did not correlate with other features typical of MPS, like lysosomal abnormalities, somatic abnormalities (like hepatosplenomegaly or osteoarticular disorders), and especially elevated GAG levels (which remained around the upper limit of normal values in these patients) [11,12,13]. Here, ARSG is linked to mucopolysaccharidosis.