ABCG2 and breast cancer: Several other investigations in different BC cohorts have demonstrated that the reduced antitumor therapy efficacy in BCSCs is also related to enhanced expression of antiapoptotic proteins, such as members of the Bcl-2 family [86], immune evasion [33] and the overexpression of ATP-binding cassette (ABC) transporters such as ABCG2 (BCRP) and P-glycoprotein, which facilitate drug efflux out the tumor cells [86].