Hydroxyurea has been approved by the FDA for the reduction of painful crises and the need for blood transfusion in homozygous SCD patients.57 It does so by increasing HbF levels, decreasing the proportion of the mutated Hb and resulting in less sickling.58 In our review, only 19.9% of the population for whom hydroxyurea use was reported took hydroxyurea (Table 4). The gene discussed is GSTM1; the disease is Schnyder corneal dystrophy.