The accumulation of muscle mass in the body is determined by the relative rates of muscle protein synthesis and degradation, namely, this process is closely related to the muscle protein turnover. Studies on the rate of muscle growth in layer and broiler chickens have shown that the rate of muscle growth are mainly determined by changes in the rate of muscle protein degradation, with little or no change in the rate of muscle protein synthesis .
Calpains, i.e. intracellular Ca2+-dependent cysteine proteases (EC 22.214.171.124), have long been demonstrated to be involved in muscle growth and development and are proenzymes that are regulated by Ca2+ binding and autoproteolytic modification . The calpains were discovered because the Z-disks in muscle strips incubated in a Ca2+-containing solution disappeared but with no other ultrastructurally detectable change . Four calpain genes (CAPN1, CAPN2, CAPN3, and CAPN1.5) are expressed ubiquitously in chicken . Study have further showed that there was less calpain activity, but more calpastatin, in broiler breast muscle . Among the calpain family members, calpain 3 (Capn3, previously named p94) is particularly interesting and has two insertion sequences, IS1 and IS2, which are involved in the regulation of its function and activity . CAPN3 was identified by positional cloning as the gene responsible for limb-girdle muscular dystrophy 2A (LGMD2A) [7, 8], where it plays an important role in myofibrillar integrity . Capn3 binds specifically to connectin (a striated muscle specific giant filamentous protein) at the N2 line , a region where proteolysis has been linked to post-mortem tenderization . CAPN3 has received considerable attention in recent years because of its potential influence on muscle growth and its role in myofibrillar organization. However, much of the available information about this gene was taken from studies on humans, mice, and other mammalian species, and might not be directly applicable to poultry.
Calpain 3 is ubiquitously expressed in chicken  and is mainly located in skeletal muscle. Compared to the other calpains, Capn3 activity is regulated by different mechanisms, as its activation and function as a muscle cytoskeleton regulator requires very low concentration of calcium and different interacting proteins . We hypothesized that CAPN3 gene may be a major gene affecting chicken carcass traits.
In this study, we screened CAPN3 single nucleotide polymorphisms (SNP) in 307 meat-type quality chickens from 5 commercial pure lines (S01, S02, S03, S05, and D99) and 4 native breeds [Huiyang Huxu chicken (HH), Qingyuan Ma chicken (QY), Caoke chicken (CK) and Mountainous black-bone chicken (MB)] in China, with an intention to evaluate potential association between CAPN3 polymorphisms and the chicken carcass traits.