ABCG2 and ovarian neoplasm: It was also shown that ABCG2 level was increased in ovarian tumor-initiating cells (TIC/CIC/CSC) and depletion of either HIF-1α or ABCG2 reduced tumorsphere (cell aggregate) formation, demonstrating HIF-1α-to-ABCG2 axis in hypoxic tumorsphere (68).