HSP90 is a molecular chaperone whose expression is elevated in cellular stress and cancer cells, and consists of HSP90α, an inducible type, and HSP90β, a constitutively expressed housekeeping type [125–127] while extracellular HSP90α [128–130] and exosomal HSP90 [122, 131–133] has been recently found. The gene discussed is HSP90AA1; the disease is cancer.