H1R-T4L was expressed in yeast Pichia pastoris. Ligand binding assays were performed as described in Methods. Pichia pastoris membranes were solubilized using 1% (w/v) n-dodecyl-β-D-maltopyranoside and 0.2% (w/v) cholesteryl hemisuccinate, and purified by immobilized metal ion affinity chromatography (IMAC). After IMAC, the C-terminal GFP was cleaved by Tobacco Etch virus (TEV) protease. Then the sample mixture was passed through IMAC to remove the cleaved His-tagged GFP and TEV protease. Receptor crystallization was performed by lipidic cubic phase (LCP) method. The protein-LCP mixture contained 40% (w/w) receptor solution, 54% (w/w) monoolein, and 6% (w/w) cholesterol. Crystals were grown in 40-50 nl protein-laden LCP boluses overlaid by 0.8 μl of precipitant solution (26-30% (v/v) PEG400, 300 mM ammonium phosphate, 10 mM MgCl2, 100 mM Na-citrate pH 4.5 and 1 mM doxepin) at 20 °C. Crystals were harvested directly from LCP matrix and flash frozen in liquid nitrogen. X-ray diffraction data were collected at 100 K with a beam size of 10 × 10 microns on the microfocus beamline I24 at the Diamond Light Source (UK). Data collection, processing, structure solution and refinement are described in Methods.