Calsequestrin, an acid glycoprotein located in the luminal space of the terminal cisternae of the sarcoplasmic reticulum, binds calcium ion and is believed to function as a storage protein for calcium. Comparison of the complete amino acid sequences of rabbit fast-twitch muscle calsequestrin and dog cardiac muscle calsequestrin, as derived from sequence cDNAs, indicate that these isoforms are clearly the products of different genes. The rabbit fast-twitch muscle calsequestrin has, in the mature protein, 367 amino acid residues. It is synthesized with a 28-residue N-terminal signal sequence that is cleaved off during synthesis. Calmitine is a mitochondrial calcium-binding protein specific for fast-twitch muscle fibers. It is absent in patients with Duchenne and Becker types of muscular dystrophy and in dystrophic dy/dy mice.