The invention relates to BALB/c mice that are deficient in the programmed cell death-1 receptor (PD-1), a screening method for autoimmune disease medicine using these mice, an IgG self-reactive antibody that the mice produce, a protein that specifically reacts to the antibody and is produced in heart, and a diagnostic method for dilated cardiomyopathy using the protein. Because PD-1 deficient BALB/c mice spontaneously develop autoimmune disease, and specifically dilated cardiomyopathy, they are useful for screening for medicines against these diseases.