Conversely, we obtained no clear evidence of a proapoptotic role for APOPT1 in available tissues (muscle, fibroblasts): a TUNEL assay was negative on muscle from individuals S1 and S2, no apoptotic bodies were observed by EM in individual S1 muscle, and no difference in apoptotic cells was found after staurosporine treatment in mutant versus control fibroblasts (not shown). However, we cannot exclude a selective apoptotic activation in other tissues/organs, for instance in brain white matter.