Disulfide bonds are rarely found in intracellular proteins, but genomic data from hyperthermophilic Archaea contradicted this well established observation from many model organisms. Intracellular proteins of hyperthermophilic Archaea, especially the Crenarchaea P. aerophilum and A. pernix, are rich in disulfide bonds, implicating disulfide bonding in stabilization of many thermostable proteins (Mallick et al., 2002).