NANOG and ovarian serous carcinoma: The small putative stem cells with diameters of up to 5 μm, expressing pluripotency-related markers NANOG, SOX2 and SSEA-4 which were identified in the ovarian sections of the women suffering from borderline ovarian cancer or high-grade serous ovarian carcinoma included into this study, were present in the ovarian surface epithelium and in special, non-defined “chambers”.