Abstract: The invention relates to polypeptides derived from the retinoic acid-related orphan receptor (ROR) in mammals, characterized in that they are delimited in their N-terminal extremity by an amino-acid located between positions 1 to 209, and in their C-terminal extremity by an amino-acid located between positions 450 to 452 of the rat ROR&bgr;, &agr;, or &ggr;, or by an amino-acid located at corresponding positions in nuclear receptor ROR of other subtypes than &agr;, &bgr; and &ggr;, and/or of the other mammals. The invention also relates to the use of these polypeptides, or of the molecular complexes or the crystals containing them, for carrying out:—a process for the screening of a ROR-LBD ligand which is an agonist, or an antagonist of said receptor,—or a process for the analysis of the tridimensional structure of the complexes formed with said polypeptides, molecular complexes or crystals and a particular compound.