Cal-520ER™ has been designed to monitor the change of calcium ion (Ca2+) in the endoplasmic reticulum (ER). Cal-520ER™ potassium salt is not cell permeable. The importance of calcium signaling in cell health and disease is the major driving force in current research of intracellular calcium homeostasis. Ca2+ release from ER and other calcium stores seems to be the crucial factor in the activation of many cellular functions. Significant changes in ER Ca2+ content and dynamics have been implicated in the activation of the ER stress response, abnormal autophagy, and cell death which leads to a variety of pathological conditions. Cal-520ER is a low-affinity Ca2+ indicator that can be used to record fast Ca2+ signals and to measure the kinetics of Ca2+ currents. Compared to Oregon Green BAPTA-5N and to Fluo4FF, Cal-520ER offers a superior signal-to-noise ratio providing the optimal characteristics for this important type of biophysical measurement. This ability is the result of a relatively high fluorescence at zero Ca2+, necessary to detect enough photons at short exposure windows, and a high dynamic range leading to large fluorescence transients associated with short Ca2+ influx periods.