An RBCC (RING finger, B-box, and coiled-coil) protein was identified that belongs to the superfamilyof zinc-binding proteins and is specifically expressed in the gill of eel, Anguilla japonica. Euryhaline fishes such as eels canmigrate between freshwater and seawater, which is considered to be accomplished by efficient remodeling of the architecture and function of the gill, a major osmoregulatory organ.