Our studies and those of others have shown that genetic factors play a key role in vulnerability to pain in cancer patients, and have identified important candidate genes such as opioid receptor genes (e.g., OPRK1 and OPRM1) [3–6], catechol-O-methyltransferase (COMT) [7, 8], and cytokine genes [9–17]. The gene discussed is OPRM1; the disease is cancer.