It implies several mechanisms, such as increased activity of efflux pumps (MRP 1/2, P-gp, BCRP), inhibition of cell death, cancer stem cells, epigenetic mechanisms, increased DNA repair, modification of drug target, inactivation of anticancer drugs, tumor cell heterogeneity, tumor microenvironment and epithelial to mesenchymal transition. The gene discussed is ABCG2; the disease is neoplasm.