Heat shock proteins or HSPs are being synthesized under different kind of stress conditions and act as molecular chaperones for protein molecules. Because these proteins were first found in cells that were exposed to high temperatures, they are called "heat shock proteins" and have been named according to their molecular weights. 

Hsp40 (heat shock protein 40 kD) is a mammalian heat shock protein. Its prokaryotic homolog is DnaJ.Hsp40 is a family of heat-shock proteins that contain a 70 amino-acid consensus sequence known as the J domain. The J domain of Hsp40 interacts with Hsp70 heat shock proteins. Hsp40 heat-shock proteins play a role in regulating the ATPase activity of Hsp70 heat-shock proteins.