We performed high-resolution mapping studies of the DNAse I-hypersensitive sites located just 5' to the human G gamma- and A gamma-globin genes of K562 erythroleukemia cells, in which these genes are constitutively expressed at low levels. This analysis revealed that the hypersensitive site extends from approximately -210 +/- 5 to -25 +/- 5 base pairs (bp) upstream from the transcription initiation site. Within this region, a GC-rich region located between the proximal CCAAT box and the TATA box is particularly accessible to nuclease digestion; however, the 5' end of the hypersensitive site is less accessible to nucleases. The pattern of DNAse I cleavage does not change on either strand with hemin induction of K562 cells, which increases the rate of gamma-globin gene transcription about threefold. The region within the hypersensitive site includes all the consensus promoter elements of the gamma-globin genes as well as an octamer sequence located between -182 and -175, and a region associated with a variety of mutations that may cause hereditary persistence of fetal hemoglobin (HPFH).