A fourth linear regression, taking the CogQOL score as the dependent variable, showed that a higher depression (B = -0.24) and insomnia severity (ISI; B = -0.17) were significantly associated with lower CogQOL scores, whereas taking an adjuvant chemotherapeutic treatment (B = 3.21) vs. a palliative regimen and having the OPRM1 AG genotype vs. AA (B = 2.05) were significantly associated with higher CogQOL scores (Table 4, Model 4). This evidence concerns the gene OPRM1 and major depressive disorder.