In vitro activity of the novel RAR agonists. HeLa cells were separately transfected with a vector expressing each isoform of the full-length receptor and a ΔMTV-IR-CAT reporter gene. The CAT activity induced by each ligand at concentrations from 50 nM to 20 μM was measured. The activity induced by the natural hormone all-trans RA at 1 μM was used as the reference for maximal induction for RARs, and 1 μM 9-cis RA was used for RXR-β. The activity plotted is the observed percentage of this maximal induction.