The CASQ2 gene provides instructions for making a protein called calsequestrin 2. This protein is found in heart (cardiac) muscle cells called myocytes, where it is involved in the storage and transport of positively charged calcium atoms (calcium ions).~Within myocytes, calsequestrin 2 is located in a cell structure called the sarcoplasmic reticulum, which acts as a storage center for calcium ions. Most of these ions are stored by attaching (binding) to calsequestrin 2. This protein also helps regulate a protein called the RYR2 channel, which controls the flow of calcium ions out of the sarcoplasmic reticulum.~For the heart to beat normally, the cardiac muscle must tense (contract) and relax in a coordinated way. This cycle of muscle contraction and relaxation results from the precise control of calcium ions within myocytes. In response to certain signals, calcium ions stored by calsequestrin 2 in the sarcoplasmic reticulum are released into the surrounding cell fluid (the cytoplasm). The resulting increase in calcium ion concentration triggers the cardiac muscle to contract, which pumps blood out of the heart. Calcium ions are then transported back into the sarcoplasmic reticulum, and the cardiac muscle relaxes. In this way, the release and reuptake of calcium ions in myocytes produces a regular heart rhythm.