Hunter syndrome, one of the mucopolysaccharidoses (MPS), results from deficiency of the lysosomal enzyme a-l- iduronidase. The glycosaminoglycans that accumulate in MPS include dermatan sulfate, heparan sulfate, keratan sulfate, and chondroitin sulfate. All of the MPS variants are autosomal recessive except for Hunter syndrome, which is X-linked recessive. Adenosine deaminase deficiency is a cause of severe combined immunodeficiency (SCID), an immunodeficiency state in which multiple recurrent infections occur after birth. Glucocerebrosidase deficiency is seen in Gaucher disease; in the most common form of the disease, there is no neurologic impairment, and patients have splenomegaly and skeletal disease as a consequence of increased lysosomal glucocerebrosides in cells of the mononuclear phagocyte system. Glucose-6-phosphatase deficiency leads to von Gierke disease, characterized by hepatomegaly, renomegaly, and impaired gluconeogenesis leading to hypoglycemia and hyperlipidemia. Hexosaminidase A deficiency occurs in Tay-Sachs disease; affected individuals manifest severe neurologic impairment, poor motor development, and blindness beginning in infancy. Lysosomal glucosidase deficiency, seen in Pompe disease, is associated with marked cardiomegaly and heart failure beginning in infancy. Sphingomyelinase deficiency occurs in Niemann-Pick disease type A, characterized by hepatosplenomegaly, lymph-adenopathy, and severe motor and mental impairment.