Pattern of interaction between mu opioid receptor SNP and baseline pain upon baseline mood score. SNP rs495491 was one of the three SNPs that showed a significant interaction term in the recessive model (p = 0.003, uncorrected for multiple tests). The baseline SF-36 MH mood scores are plotted against the baseline bodily pain scores. Each point is the mean baseline mood for all of the patients with that bodily pain score and genotype. The curve connecting the triangles represents 17 patients homozygous for the uncommon allele; the curve connecting the diamonds represents the 252 patients with one or no copies of the uncommon allele. In the presence of high baseline pain, 2 copies of the uncommon allele appear to be associated with symptoms of depression and anxiety, but the modest sample size and multiple genes tested make this result illustrative, not statistically persuasive. Data from patients with baseline pain of 0–2 (none, very mild, or mild) were pooled because few patients had such low pain scores at presentation.