The function of CASQ1 (calsequestrin 1, Ensembl gene identifier ENSG00000143318) is as follows. Calsequestrin is a high-capacity, moderate affinity, calcium- binding protein and thus acts as an internal calcium store in muscle (PubMed:28895244). Calcium ions are bound by clusters of acidic residues at the protein surface, often at the interface between subunits. Can bind around 80 Ca(2+) ions (PubMed:28895244). Regulates the release of lumenal Ca(2+) via the calcium release channel RYR1; this plays an important role in triggering muscle contraction. Negatively regulates store-operated Ca(2+) entry (SOCE) activity (PubMed:27185316). {ECO:0000269|PubMed:22337878, ECO:0000269|PubMed:27185316, ECO:0000269|PubMed:28895244, ECO:0000303|PubMed:22337878}.