Glycosaminoglycan (GAG) deposition has been identified in bone and cartilage in animal models of MPS. GAG deposition in cartilage has specifically been shown to induce inflammation (e.g. increased tumor necrosis-alpha [TNF- α ] levels in serum and synovial fluid), chondrocyte apoptosis, and hyperplasia of the synovial membranes. We have found that serum TNF-α is elevated in children and adolescents with MPS I, II and VI and is associated with bodily pain and poor physical function.