...• Histologically confirmed, UICC stage IV adenocarcinoma of the left-sided colon or rectum with metastases (metastatic colorectal cancer), primarily non-resectable, confirmed RAS mutations proven in the primary tumor or metastasis (KRAS and NRAS exon 2, 3, 4)• Age ≥ 18 years on day of signing informed consent• No previous chemotherapy for metastatic disease (1- 2 cycles FOLFIRI or mFOLFIRI are permitted before enrolment until RAS status is determined)• Patients suitable for chemotherapy administration• ECOG performance status 0-1• Consent to liquid biopsy and mutation analysis• Estimated life expectancy > 3 months• Presence of at least one measurable reference lesion according to the RECIST 1.1 criteria (chest CT and abdominal CT 4 weeks or less before enrollment)• Adequate bone marrow function defined as:o Leukocytes 3.0 x 109/L with neutrophils 1.5 x 109/Lo Thrombocytes 100 x 109/Lo Hemoglobin 9 g/dL• Adequate hepatic function defined as:o Serum bilirubin 1.5 x ULNo ALAT and ASAT 2.5 x ULN (in the presence of hepatic metastases, ALAT and ASAT 5 x ULN)• Adequate renal function: Creatinine clearance 50 mL/min• Adequate cardiac function defined aso Normal ECG and echocardiogram with a left ventricular ejection fraction (LVEF) of 55%• INR < 1.5 and aPTT < 1.5 x ULN (patients without anticoagulation). ...