SLC20A2 and glioma: Since MLV-RRV and GALV-RRV utilize different cellular receptors (PiT-2 and PiT-1 phosphate transporters, respectively) for viral entry [17], co-infection of glioma cells with both vectors supplied with different prodrug activator genes may be employed to achieve synergistic cytotoxic effects, thus augmenting the efficacy of gene therapy [18,22].