First, 13C1,2,3-lactate/13C1,2,3-glucose ratios were significantly increased in SCD but not in SCD/Sphk1−/− mouse erythrocytes in a time-dependent manner, indicating significantly increased metabolic flux through PPP in SCD mouse erythrocytes (Fig. 3f). The gene discussed is SPHK1; the disease is Schnyder corneal dystrophy.