Mutations that cause deletional severe α-thalassemia. Lines indicate uncertainty in the breakpoints. The most common mutation, the ζ‐‐^SEA mutation, shown in red, accounts for 80% of such mutations worldwide. Mutations encroaching upon the shaded area have no detectable ζ-globin in definitive erythropoiesis, despite the other promoters in the locus being deleted, suggesting a possible unidentified regulatory region.