Moreover, the tragic death of three children in a clinical trial for the treatment of X-linked myotubular myopathy (XLMTM) with an AAV8 vector delivering a functional copy of MTM1 (NCT03199469) is a sobering reminder that the injection of ever higher vector doses is not a solution for the poor transduction efficiencies of currently available AAVs. This evidence concerns the gene MTM1 and X-linked myotubular myopathy.