HBB and Schnyder corneal dystrophy: On one hand, HBB gene addition has proven to be feasible for both SCD and β-thalassemia (Thompson et al., 2018; Marktel et al., 2019), apparently leaving less room for HDR-based HBB editing (Dever et al., 2016; DeWitt et al., 2016; Kuo et al., 2018; Park et al., 2019; Pattabhi et al., 2019).