Tissue samples were collected from 10 RA and OA patients in the morning during synovectomy or surgery for complete joint replacement. The protein expression of clock genes neuronal PAS domain-containing protein 2 (NPAS2) and aryl hydrocarbon receptor nuclear translocator-like 2 (ARNTL2/BMAL2) was analyzed. The gene discussed is NPAS2; the disease is rheumatoid arthritis.