Another conspicuous discrepancy between the BU72-bound structure and the others is in helix 8 (H8). Activation of class A GPCRs, such as opioid receptors, involves an inward shift of H8, making and breaking contacts at its base (see Figures and in ). Relative to the inactive structure, the base of H8 shifts noticeably more in BU72-μOR-Nb39 than in the μOR-Gi structures or κOR-Nb39, which again cluster tightly (Figure ). This suggests that the nanobody itself is not responsible for the discrepancy, but rather some other factor, such as distortion due to strain in the adduct.