Recent studies with (PET) using nonselective (eg, [11C] diprenorphine) radiotracers or [11C] carfentanil, a selective mu-opioid receptor radiotracer, have shown reduced mu-opioid receptor availability in some chronic pain disorders, including rheumatoid arthritis,28 neuropathic pain,37 and fibromyalgia.25 This evidence concerns the gene OPRM1 and rheumatoid arthritis.