The ability to tailor biologic therapy based on the status of tumor biomarkers and monoclonal antibodies has become very important in the last years. The role of tumor biomarkers in treating colorectal cancer, specifically the K-RAS gene, was identified. K-RAS had a higher interest after Lievre and colleagues reported at the 2008 American Society of Clinical Oncology (ASCO) meeting, their analysis of K-RAS mutations in tumors from patients who did not appear to benefit from cetuximab therapy, providing additional data involving K-RAS mutant tumors and their lack of response to cetuximab, as part of first-line therapy for metastatic colorectal cancer. Furthermore, other trials evaluated the K-RAS status and the first-line treatment of metastatic colorectal cancer, the treatment of refractory metastatic cancer and dual-antibody therapy in the first-line treatment of colorectal cancer. Patients with mutant K-RAS colorectal tumors have no benefit from cetuximab, no matter the type of chemotherapy regimen.