The heat shock proteins are a highly conserved family of stress response proteins. HSPs function primarily as molecular chaperones, facilitating the folding of other cellular proteins, preventing protein aggregation, or targeting improperly folded proteins to specific degradative pathways. Some HSPs are expressed at low levels under normal physiological conditions but show dramatically increased expression in response to cellular stress, others are constitutively expressed. Specific HSPs play a role in regulating cancer by interacting directly with key components of the apoptotic pathway.