ER-Tracker Blue-White DPX dye is a photostable probe that is selective for the endoplasmic reticulum (ER) in live cells. The cells can be aldehyde-fixed after staining with the dye but significant fluorescence signal will be lost. With excitation at ∼374 nm, the environmentally-sensitive probe exhibits emission between 430 and 640 nm. A long-pass DAPI filter works best.