Mucopolysaccharidosis type VII, also known as Sly Syndrome, is a lysosomal storage disorder, characterized by deficiency of the beta glucuronidase enzyme activity. Varying forms of the condition are seen, based on its severity. Its most severe form presents in the form of hydrops fetalis or dyostosis multiplex. Other severe forms present right after birth with neonatal jaundice, joint contractures, odontoid hypoplasia, microcephaly, hepatosplenomegaly, hernias, growth retardation, and recurrent upper respiratory tract infections. In the milder form of the condition, the features remain the same, but are milder, and develop only after the child reaches about 4-years of age. Moderate, non-progressive mental retardation is seen in all forms. MPS VII is one of the rarest of all forms of mucopolysaccharidosis, with less that 1 in 250,000 live births to be affected.

MPS VII is caused by a deficiency of the beta glucuronidase enzyme caused by mutations in the GUSB gene. The beta glucuronidase enzyme plays an important role in the degradation of dermatan and keratan sulfates by catalyzing the fifth step of degradation of glucosaminoglycans. When mutations in the gene cause defects in the activity of the enzyme, the undegraded mucopolysaccharides build up in the body tissues, and cause the abnormalities associated with the condition. The range of varying phenotypic spectrum associated with the disease is due to different mutations and the corresponding residual enzyme activity.