DNA at the end point of the gene deletion associated with one form of hereditary persistence of fetal hemoglobin (HPFH) was cloned and used as a probe in gene mapping experiments to analyze the extent and approximate 3' end points of various deletions associated with HPFH and delta beta-thalassemia. The deletions in the two known forms of deletion-type HPFH were shown to be considerably more extensive than in the two cases of delta beta-thalassemia studied. The overall extents of the deletions in the two types of HPFH were quite similar in both cases and the 3' end points were located at a minimum distance of approximately equal to 52 and 57 kilobases from the 3' extremity of the beta-globin gene. In contrast, the 3' end points of the deletions in the two forms of delta beta-thalassemia were located approximately equal to 5 and 10 kilobases to the 3' side of the beta-globin gene. The extent of these deletions and the nature of the DNA brought into the vicinity of the gamma-globin genes by the deletions may therefore be a more important influence on the phenotype of the deletions than the specific nature of the DNA sequences that are deleted within the non-alpha-globin gene cluster as a result of the mutations.