ABCG2 and cancer: First, based on our previous research, 100 μM ATP stimulation showed maximal Ca2+ mobilization in cancer cells, causing dramatic induction of downstream signal transduction [39], in which genes that are involved in chemoresistance were significantly regulated, such as ATP-binding cassette subfamily B member 1 (ABCB1) and ATP-binding cassette subfamily G member 2 (ABCG2) [14, 40].