The retinoid acid receptor (RAR) and retinoid X receptor (RXR) families belong to the large group of ligand-activated transcription factors. There are three members (isotypes) in the RAR and RXR families (RARα, -β, -γ, i.e., NR1B1-3; RXRα, -β, -γ, i.e., NR2B1-3), coded by six separate genes, and different splice variants (isoforms) are known for all of them. Similar to peroxisome proliferator-activated receptors (PPAR), vitamin D receptor (VDR), thyroid hormone receptor (TR), farnesoid X receptor (FXR) and liver X receptors (LXR), the RAR forms RAR/RXR heterodimers and binds to retinoic acid-responsive elements (RARE) in the regulatory sequences of genes. The natural ligand of RAR is the vitamin A derivative all-trans retinoic acid (RA). A number of natural substances were shown to bind to RXR and activate RXR/RXR homodimers, including 9-cis RA, 9-cis dihydro-13,14-RA, and n-3 polyunsaturated fatty acids.