X-ray crystallography. Crystals were grown by the sitting drop, vapour diffusion method at 20°C. Crystals of CNGA1#621-690 grew using a 1:1 mixture of protein and reservoir solution containing 18.2% (w/v) PEG 3350, 45 mM Zn 2 + acetate, 9 mM CoCl 2 . These conditions produced crystals within 8 days with space group P2 1 2 1 2 1 that diffracted to a resolution of 2.14 Å. Crystals of CNGA3#626-672 grew using a 1:1 mixture of protein and reservoir solution containing 20% (w/v) PEG 3350, 200 mM K + acetate. These conditions produced crystals within 1 day with space group P12 1 1 that diffracted to a resolution of 1.9 Å.