The phenotypic variation within MPs and between MPs and GSHPs may also reflect increased expression of other homologous proteins such as utrophin [28], the influence of extrinsic factors (e.g. nutrition, exercise and environmental stress) or unknown breed-specific differences in the effects of dystrophin deficiency [48]. The gene discussed is UTRN; the disease is neuromuscular disease caused by qualitative or quantitative defects of dystrophin.