ABCG2 and ovarian cancer: Expression of stem cell markers (OCT4, SOX2, NANOG, NESTIN, ABCG2, CD133 and CD117) in ovarian cancer (sphere cells) represent their tumorigenic potential, and resistance to cisplatin, paclitaxel, adriamycin and methotrexate, which highlights the significance of “stemness” of cancer cells [17].