PLK1 and glioblastoma: In agreement with our study, Higuchi et al. (2018) showed a reduction in tumor growth in the xenograft model of MMR-deficient-TMZ resistant glioblastoma but also reported disparate activities of volasertib (and other PLK1 inhibitors) in xenograft versus orthotopic models suggesting that BBB penetration may be a therapeutic impediment [10].