Correlation of brain Comt and Gnb1 expression with morphine efficacy. Comt and Gnb1 expression in BXD RI strains were correlated with the slope of the morphine dose-response curve (a, n = 23) (derived from ), with the analgesia observed with 16 mg/kg of morphine (b, n = 22) (analgesia is expressed as percent of maximum effect and is derived from Fig. 1 of ), or with morphine consumption in a two-bottle choice study (c, n = 22)  in the BXD RI strains. All expression and behavioural data can be found on the WebQTL database . Comt expression was assessed using the single associated probe set (98535_at), while Gnb1 expression was assessed using probe set 94854_g_at. Rank order correlations were: Top, -0.535, P = 0.0076; middle, -0.470, P = 0.026; and, bottom, -0.586, P = 0.0035.