Calcium Ionophore I (ETH 1001) is a highly selective Ca2+ ionophore designed for biological membranes. It functions by forming stable complexes with calcium ions, facilitating their transport across lipid bilayers. This compound is extensively utilized in the development of Ca2+-selective microelectrodes, which enable precise quantitative measurements of intracellular resting Ca2+ activities and the monitoring of slow kinetic calcium fluctuations. Its high selectivity makes it an indispensable tool for investigating ion homeostasis and signaling in various cellular models.