To achieve cure is difficult and progression after response to treatment is common for patients with several type of malignancies. One possible reason is existence of cancer stem cells, which have self-renewal activity and resistance to chemotherapy and radiotherapy. Several cellular markers and signal pathways are investigated to identify cancer stem cell. Targeting cancer stem cell is considered as promising to achieve cure in combination with other treatment. Although no cancer stem cell specific treatment is established at the present moment, several targeting agents are under development. We conducted a phase I study of specific inhibitor of xCT, a glutamate-cystine transporter, of CD44 splice variant (CD44v) expressed cancer stem like cell to confirm the mode of action. Clinical study of combination therapy with cisplatin is ongoing.