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Background Adenoviral gene transfer to adult skeletal muscle is hindered by several major limitations, including host immune responses and maturation-dependent loss of myofiber infectivity. Ex vivo gene delivery is more efficient than direct viral injection in surmounting maturation-dependent adenoviral transduction. Here we investigated the use of helper cells to improve the efficiency of ex vivo gene transfer to adult mouse skeletal muscle. Methods New producer cells carrying the E1 gene of adenovirus type 5 (E32 cells) were developed using primary myoblasts from mdx mice. The E32 cells and 293 cells were infected with an E1-deleted first-generation adenovirus carrying the LacZ gene. These transduced helper cells were injected into the skeletal muscle of adult mdx and SCID mice. Results LacZ-positive mature myofibers were detected in the skeletal muscle of adult mice sacrificed 5 days post-injection. The gene transfer efficiency using 293 cells and E32 cells was 6.2 and 3.6 times higher than myoblast-mediated gene transfer, respectively. Ex vivo gene transfer of these cell types led to a better outcome than did direct adenoviral injection. Conclusions We achieved more efficient adenoviral gene transduction by using 293 and E32 helper cells than by myoblast-mediated gene transfer and direct viral injection. These helper cells also enabled adenoviral gene transfer to mature myofibers. The mechanisms by which this method facilitated adenoviral gene transfer to mature myofibers remains unclear; however, we hypothesize that the in vivo occurrence of cytopathic effects (CPE) in the transduced 293 and E32 helper cell populations facilitated the improved adenoviral transduction of myofibers. Copyright © 2004 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]