Human embryos that get too much or too little retinoic acid, a derivative of Vitamin A, can develop into babies with birth defects. New research published this week in the open-access journal PLoS Biology shows for the first time how embryonic cells may regulate levels of retinoic acid, providing insight into how retinoic acid signaling interacts with other key developmental signals to control development of the brain, limbs, and many other tissues in embryos.