Hsp90 (heat shock protein 90) is a chaperone protein that assists other proteins to fold properly, stabilizes proteins against heat stress, and aids in protein degradation. It also stabilizes a number of proteins required for tumor growth, which is why Hsp90 inhibitors are investigated as anti-cancer drugs.

Heat shock proteins, as a class, are among the most highly expressed cellular proteins across all species. As their name implies, heat shock proteins protect cells when stressed by elevated temperatures. They account for 1–2% of total protein in unstressed cells. However, when cells are heated, the fraction of heat shock proteins increases to 4–6% of cellular proteins.

Heat shock protein 90 (Hsp90) is one of the most common of the heat-related proteins. The "90" comes from the fact that it weighs roughly 90 kiloDaltons. A 90 kDa protein is considered fairly large for a non-fibrous protein. 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.

This protein was first isolated by extracting proteins from cells stressed by heating, dehydrating or by other means, all of which caused the cell’s proteins to begin to denature. However it was later discovered that Hsp90 also has essential functions in unstressed cells.

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.

In mammalian cells, there are two or more genes encoding cytosolic Hsp90 homologues, with the human Hsp90α showing 85% sequence identity to Hsp90β. The α- and the β-forms are thought to be the result of a gene duplication event that occurred millions of years ago.

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