A number of conditions that are damaging to the proteins in cells can trigger the increased expression of several highly conserved proteins that are referred to as heat shock or heat stress proteins (Hsps). Correspondingly, these proteins protect essential cellular processes that are dependent on the state of cellular proteins. Most, if not all, of these proteins are also expressed in the absence of stress and have been shown to perform critical functions in the nonstressed cell. The functions of these proteins include acting as molecular chaperones, which can prevent untoward interactions between unfolded proteins that may take place in several major cellular processes, such as protein transport, translation, and protein folding. Additionally, they can act to inhibit the irreversible aggregation of denatured proteins and, in some instances, function in the refolding of denatured proteins (Gething and Sambrook 1992; Becker and Craig 1994). It is well understood that the major Hsp families are evolutionarily conserved. The Hsps of mammalian cells can be classified into several families of sequence-related proteins, which are designated by their molecular sizes (in kilodaltons). In order of molecular size, the major families are as follows: the Hsp25/Hsp27/Hsp28 family (the small stress proteins), the Hsp40 family, the Hsp60 family, the Hsp70 family, the Hsp90 family, and the Hsp110/SSE family. The Hsp110 family does not include Hsp104, which is clearly distinguishable from Hsp110 family members and appears to be restricted to lower eukaryotes. The stress proteins of eukaryotes are present in all the