Encapsulin nanocompartments, or encapsulin protein cages, are spherical bacterial organelles roughly 30 nm in diameter that are involved in various aspects of metabolism, in particular iron sequestration which protects bacteria from oxidative stress. In terms of both structure and function, they are quite similar to ferritins. They are composed of several different encapsulin proteins, including the shell protein EncA and the internal proteins EncB, EncC, and EncD. Their structure has been studied in great detail using X-ray crystallography and cryo-electron microscopy.