Role of PPARβ/δ in the modulation of cell growth of mouse primary keratinocytes by GW0742 or retinoic acid. Primary keratinocytes from wild-type +/+) and PPARβ/δ-null (−/−) mice were treated with the indicated concentration of either GW0742 (A), atRA (B), or 9-cis RA (C; arrow), and cell number was quantified as described under Materials and Methods. Values represent the mean ± S.E.M. *, significantly different values (P < 0.05) from vehicle, as determined by ANOVA and Bonferroni’s multiple comparison test.