SPHK1 and Schnyder corneal dystrophy: To test this hypothesis, we measured the O2 equilibrium curve (OEC) by calculating the partial pressure of O2 required to produce 50% Hb-O2 saturation (P50), and found increased Hb-O2 binding affinity and thus reduced P50 in SCD/Sphk1−/− mouse erythrocytes (Fig. 3d).