Nuclear receptors: the evolution of diversity.

Nuclear receptors are an ancient family of transcription factors. Some receptors are regulated by small lipophilic ligands, whereas others are constitutive transcriptional activators or repressors. The evolution of this diversity is poorly understood, and it remains an open question as to whether or not the ancestral receptor was ligand-regulated. The recent cloning, from a snail, of an estrogen receptor that does not bind estrogen not only suggests that the steroid receptors are much more ancient than previous thought, but also points toward a mechanism through which nuclear receptors can lose the ability to be ligand regulated.