Diffraction data for the CGRPmut- and AM-bound receptor complexes were collected to resolutions of 2.5 and 1.8 Å, respectively (Table 1). The structures were solved by molecular replacement (MR) and refined to good Rwork and Rfree values (Table 1). Three copies of the tethered CGRP receptor fusion and one copy of the tethered AM1 receptor fusion were present in the asymmetric units. Molecule A (Mol A) of the CGRPmut-bound structure had the best electron density and lowest B-factors (Table 1); unless otherwise noted the figures use Mol A. The peptide-bound structures are shown in Figures 1A and 1B. The mFo-DFc electron density maps for the rebuilt MR models showed clear, unambiguous density for CGRPmut and AM (Figures S2A and S2C). MBP sits over the bound peptides, but it does not appear to alter their binding (Figures S2B and S2D). The final models include CGRPmut residues 27–37 and AM residues 35–52 (residues 25–34 were disordered) and the majority of the tethered fusion proteins other than the tethers and ∼20 residues at the C terminus, which were disordered (Figures S2B and S2D). The tethers appeared to be longer than necessary, making it unlikely that they altered RAMP-CLR interactions.