Monoclonal antibodies against BEST1WT were raised in mice by the Monoclonal Antibody Core Facility of Memorial Sloan-Kettering Cancer Center and selected for cocrystallization aids as described (Kane Dickson et al., 2014 (link)). The antibodies were expressed in mouse hybridoma cells, purified by ion exchange chromatography, and cleaved using papain (Worthington) to generate Fab fragments. The Fab fragments were purified using ion exchange chromatography (Resource S; GE Healthcare), dialysed into buffer containing 20 mM Tris-HCl, pH 7.5, 75 mM NaCl, 75 mM KCl, and further purified using SEC (Superose 6 increase 10/300 GL; GE Healthcare) using the same buffer.