While efforts are being made to identify the mechanisms of resistance and to develop novel drugs and therapeutic agents to overcome them the lack of physiologically relevant drug resistant cancer model systems remains a significant challenge drug resistance in cancer comes in two forms intrinsic resistance and acquired resistance. The major mechanisms of resistance include tumor evolution and tumor heterogeneity acquired secondary genetic alterations signaling pathway feedback loops and bypass mechanisms and the tumor microenvironment 45 drug resistance in each case could involve one or more of these major mechanisms depending on the class of drugs and therapeutic treatment strategies being employed 56. An accessible and sophisticated account of the biological mechanisms of drug resistance in cancer and their mathematical modelling book description drug resistance in cancer whereby a proportion of cancer cells evades chemotherapy poses a profound and continuing challenge for the effective treatment of cancer. That is why combining treatments that have different mechanisms of action can kill more cancer cells and reduce the chance that drug resistance will emerge most of the research on drug resistance has focused on identifying genetic mechanisms such as mutations that alter a protein such that it impairs the binding of a drug research is revealing the importance of additional mechanisms of drug resistance such as epigenetic factors that regulate the activity of genes and the dynamics between