ABCG2 and cancer: The independent and combined effects of mechanisms such as drug inactivation, drug target alteration, drug efflux (ABC transporters), DNA damage repair, inhibition of cell death, epithelial to mesenchymal transition (EMT), cancer cell heterogeneity, and epigenetic modification (DNA methylation, histone modification, etc.) significantly reduce the effectiveness of cancer drugs (Housman et al., 2014) (Figure 1).