Histone modifications within the β-globin locus in primary primitive erythroid cells at E10.5 and E12.5 indicate that the ϵy- and βH1-globin genes are contained within a single, large, hyperacetylated domain. The β-globin locus is represented to scale at the top, with the active globin genes indicated by arrows and exons of these genes by the thickest portions of the line. Unlabeled, thicker lines represent the positions of β-globin pseudogenes. The positions of PCR-amplified regions analyzed in the ChIP assay are indicated immediately below. Shown in bar graph format are enrichments, relative to multiple control sequences derived from loci containing genes inactive in erythroid cells, of cross-linked chromatin samples immunoprecipitated using antiserum against histone H3 acetylated at lysines 9 and/or 16. Chromatin was isolated from peripheral blood at E10.5 (□) and E12.5 (▪). The border between shaded and unshaded regions in each graph represents relative enrichment of 1.0-fold (arrowhead). Values for enrichment greater than 20-fold have been truncated in this representation. Error bars represent SEM.