[3H] DAMGO binding was performed in membranes of CHO cells stably expressing the indicated variant constructs. The binding parameters were determined by nonlinear regression analysis. Results are the mean ± S.E.M. of at least three independent determinations. P values determined by one-way ANOVA were 0.0013 for KDand 0.0311 for Bmax. Tukey post hoc analysis determined that rMOR-1i3 was different from rMOR-1, rMOR-1i1, rMOR-1i2 and rMOR-1H2 (p < 0.001), and that there was no significant difference among the variants in Bmax.