In vivo footprinting of bases −108 to −180 of the CD20 promoter. (A) Naked genomic DNA (lane 1) or intact cells from the B-cell line HS-Sultan (lane 2), T-cell line Jurkat (lane 3), and cervival carcinoma cell line HeLa (lane 4) were exposed to DMS, and in vivo footprints were visualized by ligation mediated PCR using primer set 3. The numbers on the left correspond to the sequence of the CD20 promoter shown in Fig 1D. The protected residues were designated as site 2 and are surrounded by a bracket. (B) In vivo footprinting was performed in an identical manner using the following cell lines: lane 1, CD20-positive plasmacytoma cell line HS-Sultan; lane 2, CD20-negative pre–B-cell line PB697; and lane 3, CD20-positive B-cell line BJA-B.