The mechanisms of MDR are very complex, and the main molecule-biological mechanisms of MDR in tumor cells were as follows: (1) drug efflux mediated by transport proteins [14,15], such as MRP, LRP, BCRP, and P-gp; (2) MDR mediated by enzymes [16], including topoisomerase (TOPO), glutathione-S-transferase (GST), and protein kinase C (PKC); (3) MDR mediated by apoptosis-associated genes [17], such as the Bcl-2 gene family, c-myc, p53, and others; (4) DNA damage repair [18]. Here, ABCG2 is linked to neoplasm.