In order to determine if GCV treatment had effectively eliminated all tumor-initiating cells or, alternatively if remaining cells could restart tumors, mice were inoculated with U87-G-P/CD133-R-R-tTK or U87-G-P/OCT4/SOX2-R-R-tTK cells and treated or not with GCV (n = 5/group). Here, POU5F1 is linked to neoplasm.