Transient mouse CPO gene promoter activity in MEL cells in the presence or absence of DMSO. MEL cells were transfected with the series of 5′-deletion mutants shown in Fig 3 and incubated in the presence or absence of 2% DMSO for 20 hours. The results are expressed as ratios to the luciferase activity of pGL3-Basic plasmid in the cells without DMSO treatment. Data are means of three separate experiments.