This pathway is used by filamentous proteins like actin and tubulin ( Johnson & Craig 1997) but also other types of proteins. An alternative pathway, independent of HSP70 or HSP90, involves binding to CCT/TriC (chaperonin containing T-complex polypeptide/ TCP-1 ring complex) proteins. proteins in translation dependent on the stage of maturation) are first bound to Hsp70, then to chaperonins ( Johnson & Craig 1997), then to more specialized proteins of the Hsp90 group ( Hartl 1996 Johnson & Craig 1997). One model suggests that client proteins (e.g. Several types of chaperones may act together, dependent on the type of protein or type of damage. of mitochondria or endoplasmic reticulum). HSPs are also involved in protein transport across membranes (e.g. Indeed, with the help of protein-protein interaction (PPI) studies, proteins could be identified as interaction partners that contribute to the following processes: transcription, mRNA splicing, translation, cell cycle control, DNA repair, apoptosis, intracellular transport, development, immune response, lipid and carbohydrate metabolism, cellular signaling, protein modification and many more ( Gong et al. Because also many undamaged proteins need assistance in folding, nearly all physiological processes require HSPs. Since HSPs stabilize DNA binding proteins it is not surprising to detect genomic instability in HSP70 deficient mice ( Hunt et al. Of course, HSPs are strongly induced by DNA damage since this type of stress leads to mutations that often interfere with proper protein folding ( Fornace et al. Meanwhile it is recognized that HSPs are involved in the response to all kinds of stress reactions that disturb proper protein conformation such as reactions to chemicals like ethanol, arsenite, cadmium, zinc, copper, mercury, sulfhydryl reagents, calcium ionophores, steroid hormones, chelating agents, viruses and many more ( Lindquist 1986). Originally, HSPs were described as proteins that were up-regulated after elevated temperatures ( Lindquist 1986). This review focuses on recent studies on HSP90AB1, if possible in comparison with its close homologue HSP90AA1. As such, they are also targets for new therapeutic approaches in cancer treatment. Since client proteins can be mutant proteins that would be degraded without the help of chaperones, HSPs also promote tumor formation and cancer cell proliferation. HSPs are necessary for a large scale of cellular processes and therefore essential for cell survival. The cochaperones influence many functions including client binding, ATPase activity or ATP binding of HSP90. HSP90 proteins act as dimers and bind clients with the help of co-chaperones. Client proteins belong to various protein families including kinases, ubiquitin ligases and transcription factors. Chaperones, by binding to client proteins, support proper protein folding and maintain protein stability, especially after exposure to various kinds of cellular stress. HSP90AB1 (heat shock protein 90 kDA alpha, class B, member 1), also known as HSP90beta, is a member of the large family of HSPs which function as molecular chaperones.