DNase I sensitivity assay of the β-globin locus. A map of mouse (A) and human (B) β-globin locus; genes are shown as ▪ and the location of LCR HS is indicated by ↓. Amplified regions used for PCR-based DNase I and ChIP assays are indicated by ▦. (C-I) PCR-based DNase I analysis. Each point of the curves represents samples of comparable molecular weight (ranging from 12 kb to 0.5 kb). For βmaj/ZFP and tg-huβ/ZFP, an additional DNase I–treated sample of 150-bp average size was included. The intensity of the PCR products was quantitated by Phosphorimager and plotted on graphs relative to the input (DNase I–untreated chromatin). Y-axis indicates relative amount of PCR products (globin relative to ZFP37 or pax6 products); X-axis, increasing DNase I concentration, ranging from 0 up to 0.35 U (see “Materials and methods”). Standard errors of mean (SEM) are indicated by vertical lines and are the results of at least 3 independent experiments. The P value was obtained using the unpaired Student t test. ZFP indicates ZFP37; βmaj, β major promoter; tg-huβ, human β promoter in transgenic ln2; mHS3, murine HS3; tgHS3, human HS3 in transgenic ln2; huβ, human β promoter in human bone marrow; and HS3, human HS3 in human bone marrow.