We identify and characterize a novel beta 0-thalassemia mutation that is associated with an unusually high level of hemoglobin (Hb) A2 in the heterozygote. This newly discovered mutation is caused by a 532-basepair deletion that extends from positions -454 to + 78 relative to the mRNA cap site of the beta-globin gene. The propositi are 9-month-old fraternal twins. One of the twins is a compound heterozygote for the deletion and Hb S, the other is a compound heterozygote for the deletion and Hb C.