However, it is only effective in approximately 10–20% of patients with colorectal cancer, with other patients showing cetuximab resistance due to gene mutations in downstream EGFR effectors, including phosphoinositide-3-kinase catalytic subunit alpha (PI3KCA), phosphatase and tensin homolog (PTEN), KRAS, and BRAF [164,169]. The gene discussed is KRAS; the disease is colorectal cancer.