Two preclinical animal studies have shown that the peripheral effects of opioids in RA is anti-inflammatory—one study was focused on kappa opioid receptors (Binder and Walker, 1998), and the other study administered endomorphin (MOR agonist) short-term and lacked adequate control groups (Straub et al., 2008). The gene discussed is OPRK1; the disease is rheumatoid arthritis.