The human wild-type CysLT1R DNA was codon optimized for insect cell expression and cloned into a modified pFastBac1 vector (Invitrogen) containing an expression cassette with a hemagglutinin (HA) signal sequence, followed by a Flag tag and a 10× His tag at the N terminus. Tags were separated from the receptor sequence by the tobacco etch virus (TEV) protease cleavage site. To facilitate crystallization, a thermostabilized apocytochrome b562RIL (BRIL; PDB ID 1M6T) was fused into ICL3 of CysLT1R (K222–K223 with S and SG linkers, respectively) with the intact N terminus and the C terminus truncated after K311. A complete DNA sequence of the crystallized CysLT1R construct is provided in Supplementary Materials and Methods.