Members of the Hsp70 family are involved in folding of newly synthesized and misfolded proteins, solubilization of protein aggregates, degradation via the proteasome and autophagy pathways, transport of proteins through membranes, and assembly and disassembly of protein complexes. Additionally, they are implicated in regulatory processes, involving the interaction with clients of the Hsp90 system, regulation of the heat shock response both in prokaryotes and eukaryotes and regulation of apoptosis. Not surprisingly, Hsp70 chaperones have therefore been linked to numerous diseases, in particular folding disorders like Alzheimers disease or Corea Huntington and many types of cancer. All different functions of Hsp70s are achieved by a transient interaction of the chaperone with substrate proteins via its Danusertib Cterminal substrate binding domain. This interaction is allosterically controlled by the nucleotide bound to the N-terminal nucleotide binding domain. In the nucleotide-free and ADP bound state the affinity for substrates is high but substrate association and dissociation rates are low. ATP binding to the NBD increases association and dissociation rates by orders of magnitude, thereby decreasing the affinity for substrates. The Hsp70 cycle is in addition controlled by the action of co-chaperones, including J-domain proteins and nucleotide exchange factors. J-domain proteins in synergism with substrates stimulate the low intrinsic ATPase activity of Hsp70 and, thereby, facilitate efficient substrate trapping. Nucleotide exchange factors accelerate the release of ADP and subsequent ATP-binding triggers substrate release. All eukaryotic cells contain several Hsp70 isoforms. In mammalian cells the most important Hsp70s are the constitutively, highly expressed cytosolic Hsc70 and the LY-300046 heat-inducible cytosolic Hsp70, the endoplasmic reticulum resident BiP and the mitochondrial mortalin. Cancer cells seem to depend on high Hsp70 activity, possibly to buffer the effect of destabilizing mutations accumulating during cell immortalization and to counter the stress conditions resulting from the nutrient depleted, hypoxic microenvironment of the tumor. Thus, levels of the heat-inducible Hsp70 are increased drastically in a variety of human tumors and this observation often correlates with poor prognosis. Furthermore, inhibition of Hsp90, which is currently being pursued actively as anti-cancer therapy and already in clinical trials, induces the heat shock response. The resulting increase of Hsp70 levels is being made responsible for cancer cell survival and the relatively small therapeutic window of Hsp90 inhibitors. Therefore, the inhibition of Hsp70, either alone or in combination with Hsp90, is believed to be a promising path in anti-tumor therapy. Such a str