4-oxo Retinoic acid is an active metabolite of the vitamin A metabolite and retinoic acid receptor (RAR) ligand all-trans retinoic acid .¹ It is formed from all-trans retinoic acid by several cytochrome P450 (CYP) isoforms, including CYP1A1, CYP3A7, and CYP26A1.2,3 4-oxo Retinoic acid binds to RARα, RARβ, and RARγ (IC50s = 59, 50, and 142 nM, respectively, in radioligand binding assays) and induces expression of a luciferase reporter in COS-7 cells expressing RARα, RARβ, or RARγ (EC50s = 33, 8, and 89 nM, respectively).¹ It increases protein levels of cytokeratin 7 (CK-7) and CK-19 in human epidermal keratinocytes when used at a concentration of 1 µM.⁴ 4-oxo Retinoic acid (10-1,000 nM) inhibits the proliferation of MCF-7 breast cancer cells.⁵ It is teratogenic to zebrafish embryos (EC50 = 8.1 nM).⁶