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ONLINE COVER Gene Therapy Moves Mountains. Shown are T cell receptor (TCR) length distributions before and after gene therapy in Wiskott-Aldrich Syndrome (WAS) patients. Most patients with a disturbed TCR profile before gene therapy improved after gene therapy, with a more polyclonal pattern of TCR Vβ usage. In this issue, Braun et al. report that hematopoietic stem cell gene therapy for WAS is feasible and effective, but the use of γ-retroviral vectors is associated with a substantial risk of leukemogenesis. [CREDIT: CHRISTIAN BRAUN/UNIVERSITY CHILDREN’S HOSPITAL, LUDWIG MAXIMILLIAN UNIVERISTY, MUNICH, GERMANY]