Sarcalumenin, also known as SAR, is a luminal Ca2+ buffer protein with high capacity but low affinity for calcium binding found predominantly in the longitudinal sarcoplasmic reticulum (SR) of fast and slow-twitch skeletal muscles and the heart. Together with other luminal Ca2+ buffer proteins, SAR plays a critical role in modulation of Ca2+ uptake and Ca2+ release during excitation-contraction coupling in muscle fibers. SAR appears to be important in a wide range of other physiological functions, such as stabilizing Sarco-Endoplasmic Reticulum Calcium ATPase (SERCA), regulating Store-Operated Calcium Entry (SOCE) mechanisms, enhancing muscle fatigue resistance, and promoting muscle development. The SAR has a molecular weight of 160 kDa (long isoform). Due to alternative splicing, the SAR gene also encodes a 53 kDa glycoprotein variant (short isoform), which shares the identical COOH-terminal half with SAR and lacks Ca²⁺-binding capacity (PMID: 19502553, PMID: 36899851).