Other Notes Review: Retinoic acid metabolism1 Unit Definition 1 U corresponds to 1 international U acc. to Ph Eur II, 217 (1983) Biochem/physiol Actions all-trans-Retinoic acid (ATRA) is a ligand for both the retinoic acid receptor (RAR) and the retinoid X receptor (RXR). The bound RAR and RXR act as transcription factors that regulate the growth and differentiation of both normal and malignant cells. Cytochromes P450 (CYPs) catalyze the 4-hydroxylation of ATRA. Retinoic acid primes embryonic stem cells to become neurons.