In the brains of patients with MPS and of MPS IIIC mice, carboxypeptidase activity measured with the specific CTSA substrate CBZ-Phe-Leu was not reduced (Figure 5, G and H), ruling out that absence or deficiency of CTSA caused the LMC disruption and secondary NEU1 deficiency in these cases. The gene discussed is NEU1; the disease is mucopolysaccharidosis.