calpastatin is a specific inhibitor of calpains, the main proteolytic enzymes of muscle. calpastatin is encoded by the cast gene, from which four different protein isoforms can be expressed due to the existence of four different promoters. this gene has been studied as a candidate to explain genetic differences in meat tenderness. in beef cattle, several single nucleotide polymorphisms (snp) have been identified in different regions of the cast gene. some of them have been included in commercial tests because of their association with the variability in beef tenderness. recently, research has focused on the regulation of different promoters in skeletal muscle and whether they have functional polymorphisms that are associated with the variability in tenderness.