The major obstacle to the cure of cancer by pharmacological agents is resistance to these agents. Clinical responses of metastatic cancers to the most advanced chemotherapeutic agents usually range from 15 to 40%, indicating that intrinsic resistance, and acquired resistance occurs almost inevitably in those tumors that do respond. In patients with metastatic colorectal cancer, clinical resistance to a particular treatment is a clear endpoint (tumor growth), and is usually observed within 6-12 months of any given therapy. Thus, drug resistance and selecting appropriate therapeutic alternatives for drug-resistant cancer remain major dilemmas for oncologists.