Aljehani MA, et al. - This study was planned to determine the association of tumor origin with mortality in a diverse population-based data set of patients receiving systemic chemotherapy (SC) and bevacizumab or cetuximab for metastatic colorectal cancer (mCRC). Primary tumor site seemed related with response to biologic therapy (BT) in mCRC. Higher mortality was observed with right-sided primary tumor location regardless of BT type. In patients with wild-type KRAS tumors, treatment with cetuximab proved beneficial only to those with left-sided mCRC and was associated with significantly poorer survival among those with right-sided mCRC. Results highlighted the significance of stratification by tumor site for current treatment guidelines and future clinical trials.