HSP90 A generic term for a family of molecular chaperones which play a key role in protein folding and quality control for a range of client proteins. Functional HSP90s operate as dimers, have intrinsic ATPase activity, act in concert with other chaperones (e.g., HSP70) and are regulated by co-chaperones/accessory proteins (e.g., HOP, CDC37). HSP90s interact with more than 100 proteins, including kinases (e.g., Raf-1), nuclear hormone receptors (e.g., oestrogen receptor), transcription factors (e.g., P53), GPCRs (e.g., CB2 receptors) and ion channels (e.g. CFTR). In humans, the HSP90-beta isoform is constitutively expressed (i.e., at baseline), whereas HSP90-alpha isoforms are induced by stress. HSP90 plays an important role in some tumour cell types by stabilising mutated oncogenic proteins.