Hsp90 (heat shock protein 90) is a molecular chaperone and is one of the most abundant proteins in unstressed cells. It is an ubiquitous molecular chaperone found in eubacteria and all branches of eukarya, but it is apparently absent in archaea. Whereas cytoplasmic Hsp90 is essential for viability under all conditions in eukaryotes, the bacterial homologue HtpG is dispensable under non-heat stress conditions.