 Currently, the most reliable way to predict whether a colorectal cancer patient will respond to one of the EGFR-inhibiting drugs is to test for certain “activating” mutations in the gene that encodes KRAS, which occurs in 30%–50% of colorectal cancers.Studies show patients whose tumors express the mutated version of the KRAS gene will not respond to cetuximab or panitumumab.Although presence of the wild-type (or normal) KRAS gene does not guarantee that these drugs will work, a number of large studies have shown that cetuximab has significant efficacy in mCRC patients with KRAS wild-type tumors.