Human glioblastoma spheroids derived either directly from the patient (low generation) or from serial in vivo passages in the brains of nude rats (high generation), were infected with lentiviral LCMV-GP (5610 4 in 10 ml) or VSV-G pseudotyped lentiviral vectors (both 5610 4 particles in 10 ml) or with retroviral MLV-based vectors pseudotyped with LCMV-GP (1610 5 particles in 10 ml). The vectors were prepared in the same way for in vitro and in vivo experiments (see materials and methods). Both lentiviral vectors transduced patient spheroids and high generation spheroids very efficiently ( Figure 1 ). In contrast, retroviral vectors transduced only a few single cells in high generation spheroids ( Figure 1 ) and failed to transduce patient spheroids (data not shown). In conclusion, both lentiviral vectors are much more efficient in transducing human glioblastoma spheroids in vitro than retroviral vectors.