The unfolded protein response (UPR) is a cellular stress response related to the endoplasmic reticulum (ER). It is a stress response that has been found to be conserved between all mammalian species, as well as yeast and worm organisms. This article focuses on the mammalian response.The UPR is activated in response to an accumulation of unfolded or misfolded proteins in the lumen of the endoplasmic reticulum.