HSP70 is a class of highly conserved stress proteins. HSP70 is comprised of multiple members, all of which bind ATP in vitro, but are localized within different intracellular compartments. These include: a) the constitutive Hsc70 (or cognate) present within the cytosol/nucleus; b) the highly stress-inducible HSP70 present within the cytosol/ nucleus/nucleolus; c) the constitutive glucose regulated 78 kDa (or BiP) protein present within the lumen of the endoplasmic reticulum; and d) the glucose regulated 75 kDa protein present within the mitochondria. HSP70 is typically not expressed in the cell under normal growth conditions, but is expressed at high levels in the cell experiencing stress. Therefore, detection of HSP70 using an antibody specific for the inducible form is quite useful in ascertaining whether a stress response has occurred in the cell.