During the follow-up of cystinosis patients under protocol #78-HG-0093 "Use of Cysteamine in the Treatment of Cystinosis", we found that 6 of our NIH patients developed papilledema and were diagnosed with pseudotumor cerebri (PTC), whose occurrence has not been previously reported in cystinosis. The goal of this protocol is to identify the role of thrombosis susceptibility in the development of PTC in nephropathic cystinosis patients in view of our recent findings regarding genetic susceptibility to thrombosis in PTC in general. We propose a case-control study. A total of 9 nephropathic cystinosis patients who developed PTC and 9 control nephropathic cystinosis patients without PTC will be screened based upon a thrombosis susceptibility screening panel, including total homocysteine, protein C and S, antithrombin III, fibrinogen, Factor VIII, Factor IX, Factor XI levels, testing for PT, PTT, activated protein C resistance, antiphospholipid antibodies (ACA panel and Lupus AC) and screening for FV Leiden mutation, FV G1628A polymorphism, FV R2 allele, Prothrombin 20210 mutation and 5,10-methylenetetrahydrofolate reductase (MTHFR) gene C677T polymorphism in patients with severe homocysteinemia (greater than or equal to 100 micro mol/l).