MKI67 and facioscapulohumeral muscular dystrophy: The present study has identified defects that may weaken the myofibers in FSHD muscles: (i) the absence of Ki67-positive proliferating cells, (ii) the formation of abnormal muscle cell clusters, (iii) a longer time for myoblast fusion, (iv) defective alignment of nuclei (that remain in clusters) in myofibers, and (v) abnormal myofibril organization.