VPA activates luciferase expression directed by the viral LTR promoter. HeLa cells (lanes 1-6) and Jurkat T lymphocytes (lanes 7-12) were transfected with 1 μg pCRLuc reporter, which contains the HTLV-1 long terminal repeat (LTR) cloned upstream of the luciferase gene, in the absence (Basal, lanes 1-3 and 7-9) or in the presence (+ Tax, lanes 4-6 and 10-12) of 10 ng plasmid pCMVTax encoding the viral transactivator protein Tax-1. Transfected cells were cultivated for 18 hours without (lanes 1, 4, 7 and 10) or with (1 mM and 5 mM on lanes 2, 5, 8, and 11 and lanes 3, 6, 9, and 12, respectively) VPA, and luciferase activity was determined. The data (in light arbitrary units ± standard deviation) result from 3 independent experiments.