KRAS and colorectal carcinoma: Cetuximab is only responsive to wild-type EGFR- and KRAS-expressing CRC patients who represent 10–20% of all CRC patients, whereas cetuximab is not responsive to approximately 80–90% of CRC patients harboring gene mutations in downstream EGFR effectors, including KRAS, PI3K catalytic subunit alpha (PI3KCA), PTEN, and BRAF.