Mucopolysaccharidosis I is caused by a deficiency of α-L-iduronidase (IDUA; EC 3.2.1.76) and can result in a wide range of phenotypic involvement with three major recognized clinical entities: Hurler (MPS IH; MIM#607014), Hurler-Scheie (MPS IH/S; MIM#607015), and Scheie (MPS IS) syndromes. The gene discussed is IDUA; the disease is isolated hemihyperplasia.