The Corfu delta beta zero thalassemia is characterized by the clinical picture of thalassemia intermedia. In the homozygous state there is a complete absence of hemoglobin (Hb) A and Hb A2 and a high level of Hb F. A DNA fragment containing the gamma and beta globin genes has been cosmid cloned, and the deletion breakpoint region, the beta globin gene and the promoter regions of the gamma globin genes sequenced. The deletion removes 7,201 base pairs (bp) containing part of the delta globin gene and sequences upstream. The beta globin gene contains a G----A mutation at IVS 1 position 5. The gamma globin gene promoters are normal. Analysis of the transcription of the mutated beta globin gene in transfected HeLa cells shows that normal message is produced at a level of approximately 20% compared with a normal gene, the remaining 80% being spliced at cryptic sites in exon 1 and intron 1. This indicates that the mutation in the beta globin gene is not the sole cause of the absence of Hb A in Corfu delta beta zero thalassemia. It is concluded that the 7.2 kilobase (kb) of deleted DNA contains sequences necessary for the normal activation of the beta globin gene. Possible mechanisms for the effect of the deletion on the expression of beta and gamma globin genes are discussed.