Survival as a function of tumour passaging in vivo. Shown are survival data for nude rats xenografted with GBM-biopsy tissue that were passaged in vivo for several generations. The patient number refers to cases listed in Table 1. For rats xenografted with tumour tissue from patient 20, survival was significantly shorter with passaging for 3 generations (p = 0.0029). For rats xenografted with tissue from patient 15, survival also decreased with passaging, although this was not significant (p = 0.69).