Map of the human β globin locus, β globin YACs A201F4 (150 kb) and A85D10 (230 kb), and YAC transgenic lines. The top line depicts the human β globin locus and flanking regions. The positions of erythroid-specific, developmentally stable DNase I hypersensitive sites proximal to the locus are indicated by arrows and numbered (5′) 4 to 1 (3′). Restriction sites for Sal 1 (Sa) and Sfi 1 (S) appear above the linear map. The position of each member of the human β globin gene cluster is designated by a closed square; the positions of probe sequences used to map the human genomic sequences are indicated by open squares under the restriction map. Closed rectangles at either end of the A201F4 and A85D10 YAC maps designate the TRP1 and URA3 arms, respectively, of the YAC vector. Wavy lines at the extreme ends of the restriction maps for each of the transgenic lines represent the approximate position of murine genomic DNA. Asterisks indicate that the 2 copies of the YAC have integrated in tandem.