Hb S/beta zero-thalassemia due to the approximately 1.4-kb deletion is associated with a relatively mild phenotype.

We report a relatively mild phenotype associated with two siblings who are compound heterozygotes for Hb S and a beta zero-thalassemia mutation due to a approximately 1.4-kb deletion of the 5' region of the beta-globin gene. Each is found to have unusually high levels of Hb A2 and Hb F, accounting for more than 20% of the total hemoglobin. These may interfere with intracellular Hb S polymerization, thus leading to a mild clinical course.