HBB and Schnyder corneal dystrophy: ,13 Interest in the molecular underpinnings of the switch from HBG to HBB expression, which occurs around the time of birth, has been stimulated by the recognition that elevated fetal hemoglobin (hemoglobin F [HbF]) levels benefit patients with sickle cell disease (SCD), a devastating disorder caused by a point mutation affecting the HBB gene (20A>T, Glu6Val).14