Disclosed are a protein and a gene each of which is a factor involved in the latent infection with a herpesvirus. An antibody against the factor is detected in about 50% of patients suffering from mental disorders, while the antibody is rarely detected in normal persons. A mouse having SITH-1 introduced therein develops bipolar disorder or a depression-like metal disorder. Based on this finding, it becomes possible to provide a method for determining a mental disorder objectively and a mental disorder model animal.
