Expression of dominant-negative variant proteins was induced by removal of tetracycline for 48 hrs prior to harvest by trypsinization. Nuclei from harvested cells were isolated and digested with 60-80 U/ml DNase I (Roche, Indianapolis, IN) for 3 min at 37°C. Digested DNA were incubated at 55°C with 10 μg/ml RNase A (Roche) for a few hours to overnight followed by addition of 25 μg/ml Proteinase K (Ambion, Austin, TX) and incubation at 55°C for at least 4 hrs. DNA fragments were purified by phenol/chloroform extraction and ultracentrifugation through a sucrose gradient. After purification, fragments (between 100 and 500 bp in size) were then pooled, precipitated and assembled into libraries for sequencing.