We showed a high transduction efficiency of lentiviral vectors for glioma cells in vivo, whereas retroviral vectors transduced a few scattered tumor cells near the injection track. This is in contrast to in vivo studies by others where retroviral vectors were very efficient, but as mentioned above, the models applied were non-invasive, based on cell lines cultured as monolayers [27] . Of note, the results of clinical studies using retroviral vectors showed the same low transduction efficiency as observed in our model system [12] . This finding provides further evidence that the glioma model used here has a higher predictive value for the performance of a novel therapeutic approach in the clinic than previous animal models.