This is an exploratory biomarker study. Patients with metastatic colorectal cancer will receive regorafenib, which is FDA approved in this setting, on day 1-21 of every consecutive 28 day cycle. Patients will be asked to undergo tumor biopsy at baseline and 2 weeks after starting regorafenib. Peripheral blood samples will be collected at baseline 2 weeks after starting regorafenib, after initiation of cycle 3, and every 4 weeks thereafter.