Intracellular resting [Ca ²⁺ ] and GEI-targeted mechanism. In resting conditions, cells maintain a [Ca²⁺] gradient between the cytosol ([Ca²⁺] ∼100 nM) and the extracellular medium ([Ca²⁺] ∼1.5–2 mM) and some organelles. The [Ca²⁺] of each compartment is color coded (bar on the right). The figure also includes a schematic representation illustrating the targeting mechanisms used to target GEIs to the different cell compartments.