Two families, one of Anglo-Saxon-Dutch descent, and the other, West Indian black, have an atypical beta thalassemia characterized by an unusually high level of Hb A2 in the heterozygous state. Restriction endonuclease mapping showed a deletion of about 1.35 kilobase (kb) in the 5′ region of the beta globin gene. Direct sequencing of a specific region of genomic DNA amplified by a new modification of the polymerase chain reaction defined the deletion to be 1,393 base pairs (bp) and to be the same in both families. The deletion extends from 485 bp 5′ to the mRNA CAP site to the middle of the second intervening sequence. This deletion, together with three others previously described that remove the 5′ end of the beta gene but leave the delta gene intact, are all associated with unusually high levels of Hb A2 in the heterozygous state.

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