HBD and Schnyder corneal dystrophy: Two HbS genes (HbSS) is most common and accounts for 60–70% of the SCD cases in the US.30 Other pathogenic HBB variants can be combined with HbS to cause SCD such as HbC, HbE, Hbβ0, Hbβ+, HbD, and HbO Arab.31 In our review, the genotype was available for 14,482 patients which is around 32.7% of all patients in the studies.