Specific enzyme measurement in plasma or leukocytes is the 2nd step in diagnosis; N-sulphoglucosamine sulphohydrolase (SGSH, EC 3.10.1.1) for MPS IIIA, N-acetyl-α-glucosaminidase (NAGLU, EC 3.2.1.50) for MPS IIIB, heparan-α-glucosaminide N-acetyl transferase (EC 2.3.1.78) for MPS IIIC, and N-acetyl glucosamine 6-sulphatase (EC 3.1.6.14) for MPS IIID. This evidence concerns the gene NAGLU and mucopolysaccharidosis type 3A.