Any of a group of proteins in living cells that assist newly synthesized or denatured proteins to fold into their functional three-dimensional structures. The chaperones bind to the protein and prevent improper interactions within the polypeptide chain, so that it assumes the correct folded orientation. This process may require energy in the form of ATP. Other functions include assisting the translocation of proteins across the membranes of cell organelles and binding denatured proteins under stress conditions or in degenerative disease. There are several unrelated families of chaperones, including five classes of heat-shock proteins—HSP25 (small heat-shock proteins), HSP60, HSP70, HSP90, and HSP100—chaperonins, calnexin, and calreticulin.