Hormones and their receptors regulate cell growth, differentiation and apoptosis and also play important roles in immune function. Recent studies on the subfamily of the orphan nuclear receptors known as retinoid-acid related orphan receptors (ROR) have shed important insights on the roles of this group of nuclear proteins in the development and function of the immune system. RORα regulates inflammatory cytokine production in both innate and adaptive immune system while RORγ regulates the normal development of T lymphocyte repertoire and secondary lymphoid organs. Cellular & Molecular Immunology. 2004;1(6):401-407.