 At the time, the three known nuclear receptor ligands were steroids, retinoids, and thyroid hormone, and of those three, both steroids and retinoids were products of terpenoid metabolism.Thus, it was postulated that ancestral receptor would have been liganded by a terpenoid molecule.In 1992, a comparison of the DNA-binding domain of all known nuclear receptors led to the construction of a phylogenic tree of nuclear receptor that indicated that all nuclear receptors shared a common ancestor.