In conclusion, screening for the DPP-IV activity in plasma samples from patients with different lysosomal diseases (LSDs) revealed elevated activity of this enzyme, especially in individuals affected with mucolipidosis II/III, alpha-mannosidosis and mucopolysaccharidoses types III, II and I. While higher DPP-IV activities have already been described in MPS patients, finding that this enzyme is elevated also in some other LSD patients is a new observation. This evidence concerns the gene DPP4 and alpha-mannosidosis.