Hereditary persistence of fetal hemoglobin (HPFH) is a benign condition in which the normal shutoff of fetal hemoglobin (Hb F) production fails to occur. In the G gamma beta+ type of HPFH, erythrocytes of adult heterozygotes contain approximately equal to 20% Hb F, which is almost exclusively of the G gamma-globin variety, without increased levels of gamma-globin chains from the nearby A gamma-globin gene. Unlike some forms of HPFH, no major deletions in the globin gene cluster have been found by genomic blotting in the G gamma beta+ variety. We report here a family with this condition, from which cosmid clones of the beta-globin gene cluster from the G gamma beta+ HPFH allele have been obtained. Sequencing around the fetal genes has identified a point mutation 202 base pairs 5' to the G gamma-globin gene that is present in genomic DNA of 3/3 unrelated individuals with G gamma beta+ HPFH but in none of more than 100 non-HPFH individuals. Although the mutation could represent a tightly linked polymorphism, its location in a region suggested by recent data to be important in tissue-specific control of gene expression suggests the possibility that the -202 mutation accounts for the phenotype. The sequence created resembles elements of other eukaryotic promoters known to be important for efficient transcription.