KRAS and neoplasm: Neither the published data nor our data is sufficient to conclude acquired resistance to the anti-EGFR antibody panitumumab is due to the emergence of cells harboring mutant KRAS. While circulating MT-KRAS was detected in the serum of some patients, given known tumor heterogeneity, it would not be surprising that the occurrence and expression of mutant KRAS in some tumors is heterogeneous.