Mucolipidosis II (ML II) and Mucolipidosis III (ML III) are inherited metabolic diseases classified as lysosomal storage diseases. Due to a defective N-acetylglucosamine-1- phosphotransferase, growing amounts of carbohydrates lipids and byproducts accumulate in various tissues and organs leading to characteristic deformities and organ insufficiencies. The phenotype resembles Hurler syndrome but in case of ML II with an earlier onset. Why ML III shows a more benign progression than ML II is poorly understood. Whereas in ML II death often occurs by the age of 5 to 8 years, in patients with ML III there is a great variability among patients, and individuals can survive into their fourth or fifth decade.