Spanish delta beta zero-thalassemia, a mild thalassemic condition characterized by increased level of hemoglobin (Hb) F production during adult life, is known to be due to a large deletion starting within the beta globin gene cluster and extending beyond the 3' breakpoint of any other similar deletional defects so far identified. By molecular cloning and by genomic mapping we now demonstrate that the deletion of Spanish delta beta zero-thalassemia ends at approximately 11 and 17 kilobases (kb) downstream to the 3' endpoints of black hereditary persistence of fetal hemoglobin (HPFH) type 1 and 2, respectively. As suggested by the complete characterization of this and other deletional defects involving the beta globin gene cluster, the 5' and 3' breakpoints of several deletions cluster in rather restricted DNA areas, further strengthening the idea that common molecular mechanisms may operate in causing these deletions.