When first-line chemotherapy stops controlling stage 4 colorectal cancer, treatment shifts from standard regimens to biomarker-driven precision therapies. Molecular testing unlocks immunotherapy, targeted therapy, and investigational options that extend survival and maintain quality of life. Key Takeaways Chemotherapy-refractory stage 4 colorectal cancer requires biomarker testing (MSI/MMR, RAS, BRAF) to identify personalized treatment pathways beyond standard chemotherapy MS