**1. Introduction**

Metastatic colorectal cancer (mCRC) remains a major contributor to cancerrelated morbidity and mortality worldwide. Among patients diagnosed with colorectal cancer, approximately 20–25% present with distant metastases, while another 20–35% develop metastases following curative-intent treatment for earlystage cancer [1]. The median overall survival for mCRC has improved significantly over the past few decades, increasing from 10 to 12 months for 5-fluorouracil plus leucovorin to currently beyond 30 months [2]. Improvements have been driven by advancements in surgery for metastatic disease, the expansion of chemotherapy options and the introduction of targeted therapies such as monoclonal antibodies against the epidermal growth factor receptor (EGFR) or the vascular endothelial growth factor A (VEGFA) [2]. Presently, there are 11 therapeutics approved by the United States Food and Drug Administration (FDA) for the treatment of mCRC, including 5-fluorouracil, irinotecan, capecitabine, oxaliplatin, bevacizumab, cetuximab, panitumumab, ziv-aflibercept, regorafenib, ramucirumab, and trifluridinetipiracil. The expansion of treatment options has resulted in an increased clinical need for predictive biomarkers to guide the effective use of therapy. Only a small proportion of patients will respond to any given therapy, and treatments are associated with significant toxicities and often with high financial costs.

Predictive biomarkers for anti-cancer agents are best developed prospectively as companion diagnostics during the drug development process. However, these can also be developed retrospectively through analysis of samples and data from previously conducted randomized clinical trials. Another avenue for marker discovery are longitudinal studies of patients analyzing the emergence of drug resistant tumor clones, although mechanisms of intrinsic (primary) and acquired (secondary) drug resistance may differ. Predictive markers can provide either drug sensitivity (positive prediction of response) or resistance (negative prediction of response) information depending on the biomarker-drug relationship.

There are many challenges in the biomarker development process, such as the choice of analyte (e.g. urine, blood, tissue), cancer sampling procedures (e.g. circulating tumor cells, primary cancer, metastatic lesions), technology for marker evaluation (e.g. DNA, RNA or protein) and determination of clinically relevant cut-offs. In this chapter, we review development efforts for predictive biomarkers for patients with mCRC focusing on anti-EGFR antibody therapies. Our discussion will concentrate on markers of primary drug resistance; markers of acquired drug resistance have been summarized in recent reviews [3, 4].
