Expression of pIN-KDEL reveals the dynamic morphology of the endoplasmic reticulum in live cells. COS-7 cells expressing pIN-KDEL were imaged 48 h post-transfection (Materials and Methods) and subjected to sequential recording of images at a rate of 1 frame/second (Supplemental movie 1). Image frames displayed at low magnification at intervals of 0, 30 and 240 sec (panels A-C, respectively) reveal changes in the overall ER morphology within the cell (Arrowheads). At higher magnification, select frames 0, 10 and 60 sec (panels D-F, respectively) reveal rapid, detailed changes in ER morphology including tubule formation, lengthening and ring opening and closing (Arrowheads). Scale bars: A-C, 10 μm; D-F, 2 μm.