PER1 and cancer: In contrast, there was no significant synthetic lethality induced by the triple combination of shHK2/DPI/PER in HK1+HK2+ cancer cells (Additional file 1: Figure S5C), suggesting that our shHK2/DPI/PER triple-combination treatment is selectively cytotoxic for HK1−HK2+ cancer cells and is likely to be tolerated by normal tissues.