Study Evaluating Biomarkers in Patients With Colorectal Cancer and Native KRAS Treated With Chemotherapy + Cetuximab

Inclusion Criteria: * Male or female, age ≥ 18 years * Able to sign an informed consent form * Advanced and/or metastatic colorectal cancer * Colorectal cancer with KRAS wild type genotype * At least one unidimensionally measurable lesion according to RECIST criteria (1.1 revised) (to be assessed ≤ 28 days prior to the study treatment) * All patients with the following features will be included: 1. Progression free survival \> 6 months after adjuvant treatment +/- radiotherapy 2. "De novo" diagnosis of the disease * Performance ECOG status of 0-2 * Life expectancy ≥ 3 months * Adequate bone marrow function: neutrophils ≥1,5 x 10\^9/L; platelets ≥ 100 x 10\^9/L; hemoglobin ≥9 g/dL. * Adequate liver, renal and hematological function as follows: 1. Adequate liver function: SGOT and SGPT 2.5 x ULN (5 x ULN in case of hepatic metastasis). Total bilirubin \< 1,5 x ULN. Alkaline phosphatase 2,5 x LSN (5 x ULN if hepatic metastasis or 10 x ULN if bone metastasis) 2. Creatinine clearance or creatinine clearance during 24 hours ≥ 50 mL/min 3. Magnesium ≥ LLN, calcium ≥ LLN Exclusion Criteria: * PS \> 2 or elderly patients with fragility criteria * Previous surgery for metastasis * Previous systemic treatment for the metastatic colorectal cancer * Previous treatment with antibodies anti-EGFR or treatment with small-molecule EGFR tyrosine kinase inhibitors or EGFR signal transduction inhibitors. Subjects who suspend their first dose due to a reaction to the infusion can participate * Central nervous system metastasis (except: treated subjects with asymptomatic CNS metastasis who have not received steroids within the 30 days prior to inclusion) * Prior malignant tumor in the last 5 years, except: basal cell carcinoma of the skin or pre-invasive cervical cancer * Unresolved toxicities from a prior systemic treatment which do not qualify the patient for inclusion * Presence of peripheral neuropathy (degree \> 1 in the ctc version 3.0) and serious nonhealing wound, ulcer, or bone fracture * Hormonal treatment, immunotherapy or experimental or approved antibodies/proteins ≤ 30 days before the inclusion * Uncontrolled serious cardiovascular disease or: congestive cardiac failure NYHA lll or lV, unstable angina pectoris, myocardial infarction precedents in the past 12 months, significant arrhythmias * Interstitial pneumonitis or pulmonary fibrosis precedents, or interstitial pneumonitis or pulmonary fibrosis signs on the thoracic CT-scan * Treatment for systemic infection within the 14 days prior to treatment * Acute/subacute intestinal occlusion and/or active inflammatory bowel disease or any other bowel disease producing chronic diarrhea * Precedent of Gilbert's syndrome or dihydropyrimidine dehydrogenase deficiency * Precedent of any disease which can increase the risks associated to the participation in the study or interfere in the study results * Known positive test for the following infections: HIV, Hepatitis C + abnormal liver enzymes values, active chronic Hepatitis B (except Hepatitis C seropositive with normal liver enzymes) * All concurrent diseases which can increase the toxicity risk * The individual presents a disorder of any kind which jeopardizes their ability to give their written consent form and/or fulfill the study procedures * Any investigational agent within 30 days before enrolment * Pregnant or breastfeeding woman, or planning to get pregnant within the 6 months after treatment * Surgery (excluding the diagnostic biopsy or placing of a central venous catheter) * Woman or man of childbearing potential not consenting to use adequate contraceptive precautions during the study and 6 months after de last administration for women, and 1 month for men * Unability to fulfill the study requirements by the patients * Psychological, family, sociological or geographical conditions that may interfere with the fulfillment of the study protocol and the follow-up calendar