Patients with chemorefractory metastatic colorectal carcinoma were enrolled into one of two panitumumab monotherapy studies (NCT00089635 and NCT00083616)10 (link). The study protocols were approved by the institutional review boards and all patients signed a written consent form. A subset of patients was selected for this analysis from a total of 388 patients. Patients received panitumumab 6mg/kg every two weeks until disease progression. Tumor scans were read centrally by a panel of at least two blinded independent radiologists using a modification of the WHO criteria. Assessments were performed at four week intervals through week 28 and every three months thereafter until progression of disease. Responses were confirmed at least four weeks after response criteria were first met. KRAS mutational status in the tissue was predetermined using the DxS assay (Qiagen).