The protein was concentrated to 20 mg ml -1 using an Amicon Ultra- 15 30 kDa (Millipore) and incubated on ice with the excess amount of androst-1,4-diene-3,17-dione (ADD) for approximately 3 hours. Subsequently, the solution was centrifuged at 14 000 g, 4° C for 5 minutes. Crystallization conditions were screened using MCSG1, MCSG2, MCSG3, Top96, SuperCOMBI, PurePEGs (Anatrace), Index, Silver Bullets, and PEG/Ion HT (Hampton Research) with success in the MCSG1 screen. Crystallization experiments were performed using the sitting-drop vapour diffusion method by mixing 1:1 screen solution with the protein solution. Finally, a diffraction-quality crystal grew in 0.1 M HEPES NaOH pH 7.5, 25% (w/v) PEG 3350 (MCSG1). For XRD experiments, the crystal was cryoprotected by drying over 1 M NaCl and flash-cooled in liquid nitrogen.