To compare the transduction efficiency of lentiviral and gammaretroviral vectors in vivo, we used a xenograft model that reflects the angiogenic and invasive features of human glioblastoma in situ. The xenograft also expresses the neural progenitor marker nestin and closely recapitulates the histology of the patient tumor ( Figure 2 ). The vectors were injected into the center of progressively growing lesions using microprocessor-controlled stereotactic infusion. The injection coordinates were estimated after analyzing MRI images for each individual lesion. The injection volume applied was 2610 ml and the vector titre 1610 7 / ml for all vectors. Transduction efficiency was evaluated 7 days after vector injection. Both lentiviral pseudotyped vectors showed very efficient transduction of the tumors ( Figure 3A,D) . When analyzed at higher magnification, both LCMV-GP and VSV-G pseudotyped lentiviral vectors showed efficient transgene delivery to nestin-positive tumor cells in solid ( Figure 3B ,E) and invasive tumor areas ( Figure 3C ,F). In contrast, the retroviral vector only transduced a few scattered tumor cells near the injection site (Figure 3 G,H) . For a quantitative comparison of transduction efficiency between the two lentiviral pseudotyped vectors, the GFP-positive areas were measured on histological slides (see material and methods). The total volume of transduced tumor tissue was 7.0563.51 mm 3 for LCMV-GP pseudotyped vectors and 4.0562.04 mm 3 for VSV-G pseudotyped vectors ( Figure 3I) . Although there was a difference in the mean, it was not statistically significant (p = 0.269) due to high interindividual differences (standard deviations).