Bex1 protein is upregulated in regenerating muscle and interacts with calmodulin, a Ca2+-binding protein involved in cell cycle regulation. Following cardiotoxin-induced injury the regenerating muscle of Bex1knock-out mice exhibits prolonged cell proliferation and delayed cell differentiation compared to wildtype mice. To gain insight into this process, we compared the regenerating myogenic morphologies of Bex1 knock-out and wild-type mice at several time points. Bex1-positive cells were identified by double immunofluorescence staining. These studies demonstrated that a population of cells that are Bex1-positive after injury are c-Met/basal lamina-positive and Mac-1-negative indicating that they are derived from at least a subset of myogenic progenitor/satellite cells but not invading immune cells. In addition, in regenerating muscle, Bex1 co-localizes with calmodulin in the cytoplasm of the late myoblast or early myotube stage of myogenesis. These results suggest that Bex1 participates in muscle regeneration through the regulation of satellite cell proliferation and differentiation by its interaction with calmodulin. Current studies of Bex1 may provide a new molecular tool for the identification of activated satellite cell and open the way to new or improved therapeutic regimens against progressive muscular atrophy.