ATRA-refractory repression of reporter gene expression by PML-RARα. A DR5-tk-luciferase reporter was transiently transfected into CV1 cells together with expression vectors for RARα, PML-RARα, or an empty vector control, as indicated. The cells were subsequently incubated with the ATRA concentrations indicated, harvested 24 h later, and assayed for luciferase activity. The means and standard errors for three independent experiments are shown.