Background: RAS mutations are the only validated biomarkers in metastatic colorectal cancer (mCRC) for anti-epidermal growth factor receptor (EGFR) therapy. Limited clinical information is available on AXL expression, marker of epithelial to mesenchymal transition, in mCRC., Methods: AXL was retrospectively assessed by immunohistochemistry in 307 patients. RAS wild-type (WT) patients (N = 136) received first-line anti-EGFR-based therapy; RAS mutant patients (N = 171) received anti-angiogenic-based regimens. Preclinical experiments were performed using human RAS WT CRC cell lines and xenograft models. AXL RNA levels were assessed in a cohort of patients with available samples at baseline and at progression to anti-EGFR treatment and in the GSE5851 dataset., Results: AXL was expressed in 55/307 tumour tissues, correlating with worse survival in the overall population (AXL-positive, 23.7 months; AXL-negative, 30.8 months; HR, 1.455, P = 0.032) and in RAS WT patients (AXL-positive, 23.0 months; AXL-negative, 35.8 months; HR,1.780, P = 0.032). Progression-free survival (PFS) in the RAS WT cohort was shorter in the AXL-positive cohort (6.2 months versus 12.1 months; HR, 1.796, P = 0.013). Three-dimensional cultures obtained from a patient following anti-EGFR therapy resulted AXL-positive, showing resistance to anti-EGFR drugs and sensitivity to AXL inhibition. AXL transfection in CRC cell lines induced AXL overexpression and resistance to the EGFR blockade. At progression to cetuximab, 2/10 SW48-tumour xenograft mice showed AXL expression. Consistently, AXL RNA levels increased in 5/7 patients following anti-EGFR therapy. Moreover, in the GSE5851 dataset higher AXL RNA levels correlated with worse PFS with cetuximab in KRAS-exon2 WT chemorefractory patients., Conclusions: AXL is a marker of poor prognosis in mCRC with consistent clinical and preclinical evidences of involvement in primary and acquired resistance to anti-EGFR drugs in RAS WT patients., Competing Interests: Conflict of interest statement TT reports advisory board for Amgen, Bayer, Merck, Novartis, Roche and Sanofi. FM reports advisory board for Lilly and MSD. ChCr reports personal fees from Roche, Amgen, Bayer and Servier, research funding from Merck Serono and consulting or advisory role for Roche, Bayer and Amgen. FP reports honoraria/speaker bureau from Amgen, Roche, Merck Serono, Lilly, Sanofi, Bayer and Servier and research grants from BMS. GWP reports personal fees from advisory boards and/or speaker's honorarium: Amgen, Bayer, BMS, Boston Biomedical, Celgene, Halozyme, Lilly, Merck Serono, Roche, Sanofi, Servier, Shire, MSD, Taiho and CECOG. NN reports personal financial interests as speaker's fee and/or advisory boards from MSD, Qiagen, Bayer, Biocartis, Incyte, Roche, BMS, Merck, Thermo Fisher, Boehringer Ingelheim, AstraZeneca, Sanofi and Eli Lilly; institutional financial interests (financial support to research projets) from Merck, Sysmex, Thermo Fisher, Qiagen, Roche, AstraZeneca and Biocartis; non-financial interests: President, International Quality Network for Pathology (IQN Path), President Elect, Italian Cancer Society (SIC). EvMa reports personal financial interests honoraria for advisory role for advisory board from: Astra Zeneca, Sanofi Genzyme, Servier, Celgene, Merck, Lilly and Roche; research grants from MSD and BSD; travel grants from Merck and Roche; institutional financial interests as financial support for clinical trials from Roche, Sanofi Genzyme, Teva and Merck. GA reports personal financial interests from Hoffman-La Roche, Bristol Myers Squibb, Bayer, Servier, Amgen, Merck Serono and Menarini; institutional financial interests from Bayer, Servier, Novartis, Boehringer Ingelheim, Boston Pharmaceuticals, Hoffman-La Roche and Genentech. EE reports personal financial interests, honoraria for advisory role, travel grants, research grants from Hoffman-La Roche, Sanofi Aventis, Amgen, Merck Serono and Servier, MSD; institutional financial interests from Hoffman-LaRoche, Sanofi Aventis, Amgen, Merck Serono, MSD, Boehringer Ingelheim, AbbVie, Array Pharmaceuticals, Pierre Fabre and Novartis. AF reports personal financial interests (advisory role/honoraria): Bayer, Bristol, Lilly, Merck, Pierre Fabre, Roche, Servier; institutional financial interests as financial support for clinical trials from AstraZeneca, Bayer, Bristol, Lilly, Merck, MSD, Novartis, Roche, Sanofi and Servier. JT reports scientific consultancy roles for Bayer, Boehringer Ingelheim, Chugai, Genentech, Inc., Ipsen, Lilly, MSD, Merck Serono, Merrimack, Merus, Novartis, Peptomyc, Pfizer, Rafael Pharmaceuticals, F. Hoffmann-La Roche Ltd, Sanofi, Seattle Genetics, Symphogen and Taiho. FC reports being in the advisory boards for Merck, Roche, Amgen, Bayer, Servier, Symphogen and Pfizer and research funding from Roche, Merck, Amgen, Bayer and Ipsen. ErMa reports being in the advisory boards for Amgen, Bayer, Merck, Roche, Sanofi and Servier and giving expert opinion for ESMO (European Society of Medical Oncology). ClCa, BB, VMV, GM, VS, PPV, DC, VB, NM, FZ, MD, AN, AB, CA, GS, RF and MS report no competing conflicts of interest., (Copyright © 2020 Elsevier Ltd. All rights reserved.)