Retinoic acid (used simplified here for all-"trans"-retinoic acid) is a metabolite of vitamin A1 (all-"trans"-retinol) that mediates the functions of vitamin A1 required for growth and development. All-"trans"-retinoic acid is required in chordate animals, which includes all higher animals from fish to humans. During early embryonic development, all-"trans"-retinoic acid generated in a specific region of the embryo helps determine position along the embryonic anterior/posterior axis by serving as an intercellular signaling molecule that guides development of the posterior portion of the embryo. It acts through Hox genes, which ultimately control anterior/posterior patterning in early developmental stages.