Wijgerde MGJM 1 , Verheijen FW 1 , Schoonderwoerd GC 1 , Ruijter GJG 1 , Huijmans JGM 1 1 Dept. of Clinical Genetics, Erasmus MC, Rotterdam, Netherlands Mucolipidosis (ML) II and III are inherited lysosomal storage disorders caused by a deficiency of the enzyme N-acetylglucosaminyl-1-phosphotransferase (GlcNac-1-phosphotransferase), whose activity is required for the post translational processing of many hydrolases to form the mannose 6-phosphate (M6P) recognition site that permits lysosomal targeting. As a result of this enzyme deficit, lysosomal enzymes are secreted by cells and abnormal amounts of carbohydrates and fatty materials (lipids) accumulate in lysosomes, resulting in damaged tissues causing symptoms that range from joint problems and mild learning disabilities to severe retardation and skeletal deformities. Biochemical analysis of ML II/III patients show increased hydrolase (eg. beta-D-hexosaminidase, beta-D-glucuronidase, beta-D-galactosidase, and alpha-L-fucosidase) activities in blood plasma and elevated excretion of oligosaccharides in urine. Quantitative analysis using LC-MS-MS of free and bound sialic acid concentrations in urine samples of ML II/III patients reveal excessive excretion of bound sialic acid in 80% of investigated patients. In contrast, mildly elevated excretion of glycosaminoglycans (GAGs) detected by spectrophotometry after dimethylmethyleneblue staining and abnormal oligosaccharides detected by thin layer chromatography were each present in 60% of our patients. Excessive urinary excretion of bound sialic acid in a metabolic screen indicates, apart from sialidosis (ML I), also ML II/III as a putative diagnosis.