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Protein complexes that mediate the release of calcium from the sarcoplasmic reticulum in both skeletal and cardiac muscle cells by forming tetrametric complexes. These complexes each then act as a calcium channel. There are three isoforms of the RyR: RyR1, RyR2, and RyR3. RyR1 is specifically expressed in skeletal muscles and RyR2 in cardiac muscles. RyR3 is yet another isoform found in non-muscle cells such as neuronal cells. (From Yonsei Med J 1996;37 (1):72-80)