by the expression of a polypeptide that converts a ligand precursor into a ligand and the ligand thus formed binds to a nuclear receptor to thereby induce the expression of a reporter gene located downstream of the target sequence is constructed searching a gene library using this system can isolate a gene encoding a polypeptide capable of converting a ligand precursor into a ligand this system which takes the advantage of the transcriptional regulatory function of a nuclear receptor enables screening a ligand that binds to a nuclear receptor and to examine whether or not a test compound is a ligand that binds to the nuclear receptor and also screening genes that encode polypeptides capable of converting an inactive form of a wide range of transcriptional regulatory factors into an active form application_number US 3069905 A description the present invention is demonstrated with reference to examples below but is not to be construed being limited thereto isolation of cdna encoding an enzyme that hydroxylates 1 u03b1 position of vitamin d the inventors developed an expression cloning system mediated by a nuclear receptor for cloning a full length cdna encoding 1 u03b1 oh ase the system is based on the mechanism that 25 oh d 3 a precursor of 1 u03b1 25 oh 2 d 3 can activate the transactivating function of vdr only in the presence of 1 u03b1 oh ase fig1 in other words the ligand dependent transactivating function of vdr af 2.