The proportion of rheumatoid factor, ACPA, SE, and PTPN22 positive twins was 67.2, 76.0, 75.2, and 30.7% among the RA twins and 7.2, 18.9, 64.2, and 23.4% among their healthy co-twins. The gene discussed is PTPN22; the disease is rheumatoid arthritis.