**6.3 Cell cycle of LSCs**

In patients with drug resistance, most of their LSCs are in the quiescent phase (G0 phase) and therefore cannot be effectively eliminated by chemotherapy. Hence, some people consider that LSCs in stationary phase can be eliminated by two-step method: (1) Stimulate LSCs from the G0 phase into the cell cycle proliferation and then use specific tumor-targeted therapeutic drugs to eliminate LSCs and (2) Let LSCs stay in the G0 phase. It is worth noting that although the cells in the G0 phase are dormant, they have the ability to proliferate; thus, this can only delay survival and avoid recurrence. Experiments in vitro have shown that cyclin-dependent kinase (CDK6) can be involved in the regulation of cell cycle, and inhibition of CDK6 may cause leukemia stem cells to dormant and inhibit cell proliferation [94].
