Disclosed are compounds or their salts that promote or inhibit the activities of a protein comprising the same or substantially the same as the amino acid sequence represented by SEQ ID NO: 1, compounds or their salts that promote or inhibit the activities of a gene for the protein, antisense polynucleotides comprising the whole or part of a base sequence complimentary or substantially complimentary to the base sequence of a polynucleotide encoding the protein or its partial peptide, antibodies to the protein, etc. These compounds or their salts, antisense nucleotides, antibodies, etc. can be used as agents for the prevention/treatment of mental disorders, prolactin-related disorders, etc.