Any of various proteins that are synthesized by living cells in response to increased temperature or other forms of stress. They occur in both eukaryotes and prokaryotes, and function mainly as molecular chaperones, protecting the cell’s proteins as they become unfolded due to heating and enabling them to refold correctly. There are several families of heat-shock proteins, named according to their relative molecular mass (in kilodaltons, kDa); the larger ones, including HSP100, HSP90, HSP70, and HSP60, tend to predominate in animal cells. The smaller ones, with molecular masses in the range 17–28 kDa, are more common in plants. Another protein, ubiquitin, is regarded as a heat-shock protein and acts as a marker for proteins destined for degradation. Many of the heat-shock proteins are also induced by other stresses, both natural and unnatural, including reduced oxygen concentration, changes in osmotic potential, ionizing radiation, and toxins, and hence are also called stress proteins.