Polyether nature products have been of special attention because of their fascinating structure and biological effects. They are widely used as anticancer, antibiotics, and antivirus therapeutics. Lasalocid A is an ionophore polyether that can be synthesized by a series of enzymes Lsd11 to Lsd19 in Streptomyces lasaliensis. It contains a polycyclic ether skeleton constructed by enantioselective epoxidation and epoxide-opening cyclization. Our group has investigated the epoxide hydrolysis step in lasalocid A biosynthesis. Now our interest is in the epoxidation step where the ether ring stereoconfiguration is determined by the epoxidase Lsd18. Solving Lsd18 atomic structure using X-ray crystallography would help us understand the mechanism of ether ring stereocontrol.