FIG. 2.

Structure of the human β-globin locus. The β-LCR extends from 6 to 18 kb upstream of the ϵ-globin gene and contains four strong erythroid-specific DNAse I-hypersensitive sites (represented by arrows) that are present at all stages of development (HS1-4). HS5 is constitutively sensitive to nuclease digestion and may function as an insulator or boundary element. Interactions between the LCR and the promoters of individual globin genes (indicated by curved arrows) are thought to be mediated by stage-specific regulatory proteins that stabilize the formation of DNA loops. The figure is not drawn to scale.