We have identified the sequences in the human gamma-globin gene promoter that are required for efficient and accurate transcription by using deletion mutants and in vitro site-directed mutagenesis. More than 131 bp (which include the 'CACA' sequence) upstream from the Cap site, are required for efficient transcription of the gamma-promoter. Furthermore, the 27 bp tandemly duplicated segment including the conserved 'CCAAT' box (represented only once in the beta promoter and required for its function) is not essential for the gamma-promoter, since scanning-linker mutants lacking part or all of the proximal 'CAT' box exhibited a 1.5 to 4.0 fold increase in the promoter function compared to the wild type. These data and the recently described point mutations in the regions of the A gamma and G gamma promoters associated with hereditary persistence of fetal hemoglobin, support the notion that the DNA sequences of the gamma-promoter region should be involved in the developmental regulation of globin genes. Molecular analysis of naturally occuring deletion-mutations in the beta-globin gene cluster leading to increased production of HbF in adult life, showed that irrespective of the nature of the deletions, they exhibit identical clinical and cellular phenotypes on interaction with the beta S gene. Erythroid progenitor cells from these individuals respond in vitro, to the same extent to the modulating effects of exogenous factors such as the 'switching activity' of fetal sheep serum with respect to their pattern of hemoglobin synthesis.