 This is a phase 2, multicenter, randomized, double-blind, double-dummy, placebo-controlled, three-arm trial to be conducted in the United States, Europe, and Asia. Approximately 150 eligible KRAS-mutant metastatic colorectal cancer subjects who have failed first line fluoropyrimidine and oxaliplatin-based regimen with or without anti-VEGF therapy will be randomized in a 1:1:1 ratio to receive AMG 479 placebo plus AMG 655 with FOLFIRI, or AMG 479 plus AMG 655 placebo with FOLFIRI, or AMG 479 placebo plus AMG 655 placebo with FOLFIRI 