Ligands, which are small and lipophilic molecules such as steroid hormones, enter cells by a passive mechanism and bind to the nuclear receptors inside cells. Ligand binding results in a conformational shift, which renders the receptors transcriptionally active. During recent years a large number of proteins have been identified that belong to the nuclear receptor family but lack identified ligands. These proteins, referred to as orphan receptors, represent exciting research areas since they indicate the existence of as yet uncharacterized signaling systems of potential importance in adult physiology and development. One of the studied orphan receptors is essential in developing and mature dopamine cells and of relevance for research on Parkinson's disease.
Additional studies focus on retinoid signaling in the developing and adult central nervous system.