As shown elsewhere [47–50], we found that metformin induces the arrest of proliferative cells (S-G2/M accumulation) and, indeed, metformin downregulated the expression of cyclin A, cyclin B1, and cdc2 in sensitive cells but not the G0/G1 checkpoint protein cyclin E. G0/G1 cell cycle arrest has been reported to be induced by metformin in glioma, ovarian, and endometrial cancer cells [37, 51, 52], suggesting the specificity of metformin's antiproliferative action [47]. Here, CCNE1 is linked to glioma.