DNase I hypersensitive sites of GFP transgenes. (A) Southern blot of two C-GFP and two A-GFP subclones. DNA from DNase I–digested cells was digested with Pst I and Bcl I and hybridized with a GFP probe. 0, 10, and 50 U DNase I are indicated by the filled horizontal arrows. (B) Ethidium bromide–stained gel of DNA fragments before hybridization for A. (C) An ovalbumin probe was used to reprobe the blot in A. (D) Vertical arrows indicate DNase I hypersensitive sites observed with GFP for C-GFP and A-GFP clones. The horizontal blue box indicates the probe. The experiment was repeated with DNase I digestion of different C-GFP and A-GFP cell clones showing no difference in DNase hypersensitivity.