The sarcoplasmic reticulum (SR) is, in part, responsible for maintaining the level of intracellular calcium in cardiac and skeletal muscle by storing and releasing calcium. Several intralumenal SR calcium binding proteins have been identified, the most prominent of these is calsequestrin. Calsequstrin is a calcium binding protein known to sequester calcium accumulated in the sarcoplasmic reticulum of muscle cells during relaxation and is found discretely localized to the junctional and corbular (terminal cisternae) SR. Calsequestrin functions to localize calcium near the junctional face of the terminal cisternae from which calcium can be released into the cytosol via the ryanodine receptor. This protein is highly acidic and has a large capacity and moderate to low affinity for calcium.