This randomized, open-label study will evaluate the safety and efficacy of Avastin (Bevacizumab) added to standard mFOLFOX6 chemotherapy in treatment-naïve patients with Stage IV metastatic colorectal cancer. According to K-RAS gene mutation status, patients will be assigned or randomized to receive either Avastin 5 mg/kg intravenously (iv) on Day 1 of each 2-week cycle or cetuximab 400 mg/m2 iv on Day 1 followed by 250 mg/m2 iv every week, in addition to mFOLFOX6 every 2 weeks. Anticipated time on study treatment is until disease progression or unacceptable toxicity occurs.