The folding of polypeptide chains in cells, following either translation or translocation through membranes, must take place under conditions of extremely high protein concentrations. In addition, folding into a correct structure must occur in the presence of other rapidly folding species, and at temperatures known to destabilize aggregation-prone folding intermediates. To facilitate folding in vivo, molecular chaperones have evolved that stabilize protein folding intermediates, thus partitioning them towards a pathway leading to the native state rather than forming inactive aggregated structures.