Hsp90 is a molecular chaperone and is one of the most abundant proteins expressed in cells. It is a member of the heat shock protein family which is upregulated in response to stress. Hsp90 is found in bacteria 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.

Hsp90 is highly conserved and expressed in a variety of different organisms from bacteria to mammals – including the prokaryotic analogue htpG (high temperature protein G) with 40% sequence identity and 55% similarity to the human protein. Yeast Hsp90 is 60% identical to human Hsp90α.Heat shock protein 90 (Hsp90) is one of the most common of the heat related proteins.