Background: The development of anti-HER2 antibody-drug conjugates opens new therapeutic options for patients with breast cancer, including patients with low expression of HER2. To characterise this new breast cancer subtype, we have compared the clinical and molecular characteristics of HER2-low-positive and HER2-zero breast cancer, including response to neoadjuvant chemotherapy and prognosis., Methods: In this pooled analysis of individual patient data, we evaluated a cohort of 2310 patients with HER2-non-amplified primary breast cancer that were treated with neoadjuvant combination chemotherapy in four prospective neoadjuvant clinical trials (GeparSepto, NCT01583426; GeparOcto, NCT02125344; GeparX, NCT02682693; Gain-2 neoadjuvant, NCT01690702) between July 30, 2012, and March 20, 2019. Central HER2 testing was done prospectively before random assignment of participants in all trials. HER2-low-positive status was defined as immunohistochemistry (IHC) 1+ or IHC2+/in-situ hybridisation negative and HER2-zero was defined as IHC0, based on the American Society of Clinical Oncology/College of American Pathologists guidelines. Disease-free survival and overall survival data were available for 1694 patients (from all trials except GeparX) with a median follow-up of 46·6 months (IQR 35·0-52·3). Bivariable and multivariable logistic regression models and Cox-proportional hazards models were performed based on a predefined statistical analysis plan for analysis of the endpoints pathological complete response, disease-free survival, and overall survival., Findings: A total of 1098 (47·5%) of 2310 tumours were HER2-low-positive and 1212 (52·5%) were HER2-zero. 703 (64·0%) of 1098 patients with HER2-low-positive tumours were hormone receptor positive, compared with 445 (36·7%) of 1212 patients with HER2-zero tumours (p<0.0001). HER2-low-positive tumours had a significantly lower pathological complete response rate than HER2-zero tumours (321 [29·2%] of 1098 vs 473 [39·0%] of 1212, p=0·0002). Pathological complete response was also significantly lower in HER2-low-positive tumours versus HER2-zero tumours in the hormone receptor-positive subgroup (123 [17·5%] of 703 vs 105 [23·6%] of 445, p=0·024), but not in the hormone receptor-negative subgroup (198 [50·1%] of 395 vs 368 [48·0%] of 767, p=0·21). Patients with HER2-low-positive tumours had significantly longer survival than did patients with HER2-zero tumours (3-year disease-free survival: 83·4% [95% CI 80·5-85·9] vs 76·1% [72·9-79·0]; stratified log-rank test p=0·0084; 3-year overall survival: 91·6% [84·9-93·4] vs 85·8% [83·0-88·1]; stratified log-rank test p=0·0016). Survival differences were also seen in patients with hormone receptor-negative tumours (3-year disease-free survival: 84·5% [95% CI 79·5-88·3] vs 74·4% [70·2-78.0]; stratified log-rank test p=0·0076; 3-year overall survival: 90·2% [86·0-93·2] vs 84·3% [80·7-87·3], stratified log-rank test p=0·016), but not in patients with hormone receptor-positive tumours (3-year disease-free survival 82·8% [79·1-85·9] vs 79·3% [73·9-83·7]; stratified log-rank test p=0·39; 3-year overall survival 92·3% [89·6-94·4] vs 88·4% [83·8-91·8]; stratified log-rank test p=0·13)., Interpretation: Our results show that HER2-low-positive tumours can be identified as new subgroup of breast cancer by standardised IHC, distinct from HER2-zero tumours. HER2-low-positive tumours have a specific biology and show differences in response to therapy and prognosis, which is particularly relevant in therapy-resistant, hormone receptor-negative tumours. Our results provide a basis for a better understanding of the biology of breast cancer subtypes and the refinement of future diagnostic and therapeutic strategies., Funding: German Cancer Aid (Deutsche Krebshilfe)., Competing Interests: Declaration of interests CD reports grants from European Commission H2020, German Cancer Aid Translational Oncology, German Breast Group, during the conduct of the study; personal fees from Novartis, Roche, MSD Oncology, Daiichi Sankyo, AstraZeneca, Molecular Health, and Merck, and grants from Myriad, outside the submitted work. CD is the cofounder of Sividon diagnostics. CD has a patent VMScope digital pathology software with royalties paid, a patent WO2020109570A1-cancer immunotherapy pending, and a patent WO2015114146A1 and WO2010076322A1-therapy response issued. AS reports grants from Celgene, Roche, and AbbVie; personal fees from Roche, AstraZeneca, Celgene, Novartis, MSD, Tesaro, Lilly, and Roche, outside the submitted work. TL receives payment or honoraria from Tesaro, MSD, Roche, Novartis, Pfizer, Amgen, Clovis, and Lilly; receives support for travel costs from Roche, Pfizer, MSD, Celgene, and Clovis; reports participation on an Advisory Board from Roche, Tesaro, Amgen, MSD, Pfizer, Lilly, Myriad, Esai, and GSK, outside the submitted work. J-UB reports personal fees from AstraZeneca, Exact Sciences, Amgen, MSD Oncology, Lilly, Novartis, Sysmex, Roche, and Sonoscape, outside the submitted work. PW reports personal fees from Amgen, AstraZeneca, MSD, Novartis, Pfizer, PharmaMar, Roche, Teva, Eisai, Clovis, and Tesaro, outside the submitted work. HT reports consulting fees and travel costs from Pierre Fabre, Pfizer Pharma, Mundipharma, ClinSol, Novartis, Lilly, Grünenthal, Vifor, AstraZeneca, Mylan, and AMGEN; payment or honoraria from Vifor, Novartis, AstraZeneca, ClinSol, Mundipharma, Lilly, and Amgen; participation on an Advisory Board from Novartis, Bristol Myers Squibb, AstraZeneca, Molecular Health, Pfizer, Roche, Grünenthal, and Mylan; and is part of a leadership or fiduciary role in Arbeitskreis klinische Studien. ASCO, BNHO, DGHO, DKG, ESMO, Deutsche Gesellschaft für Senelogie. CJ receives payment or honoraria from Roche, AstraZeneca, Novartis, Lilly, Exact Sciences, and Pierre Fabre; recieves support for travel costs from Roche and AstraZeneca, reports participation on an Advisory Board from Roche, Lilly, and AstraZeneca. MR reports consulting fees from Roche, Novartis, Lilly, and Somatex; receives payment or honoraria from Novartis, Roche, Lilly, Somatex, and AstraZeneca; recieves support for travel costs from Novartis, Pfizer, and Celgene. EFS receives payment or honoraria from AstraZeneca, Pfizer, Celgene, Roche, and Amgen; support for travel costs from Amgen, AstraZeneca, Celgene, Clovis Oncology, Eisai, Erbe, Gedeon, Richter, Genomic Health, Jenapharm, Johnson Johnson, KLS Martin, Matramed, Medac, Mentor, Novartis, Pfizer, Pharma Mar, MSD, Roche, Samsung, Storz, Tewa, Vifor, Medconcept, and Thieme; is part of a leadership or fiduciary role of University of Saarland, Germany, society for German Gynecological Endoscopy. CH reports personal fees from Pfizer, Roche, Novartis, Lilly, AstraZeneca, Celgene, outside the submitted work. PAF reports personal fees from Novartis, Pfizer, Daiichi-Sankyo, AstraZeneca, Eisai, Merck Sharp & Dohme, Lilly, Pierre Fabre, Seattle Genetics, Roche, and Hexal, and grants from Biontech and Cepheid, during the conduct of the study. KL receives payment from Roche, Pfizer, MSD, Novartis, Lilly, and Genomic Health, support for travel costs from Roche, and reports participation on an Advisory Board from Roche, MSD, and Esai. JH reports research grants to institution from Celgene, Novartis, and Hexal; consulting fees from Lilly, Novartis, Roche, Pfizer, Hexal, AstraZeneca, MSD, Celgene, and Abbvie; receives payment from Lilly, Novartis, Roche, Pfizer, AstraZeneca, MSD, Celgene, Eisai, and Abbvie; recieves support for travel costs from Roche, Pfizer, Novartis, Celgene, and Daiichi. TR reports consulting fees from Pfizer, honoraria for presentations from AstraZeneca and receives payment from Pfizer, Roche, and Novartis. MS reports grants from AstraZeneca, BioNtech, Eisai, German Breast Group, Genentech, Novartis, Pantarhei Bioscience, Pfizer, Pierre Fabre, and Roche; consulting fees from AstraZeneca, Eisai, Lilly, Novartis, Pantarhei Bioscience, Pfizer, Pierre Fabre, Roche, and SeaGen; receives payment or honoraria from AstraZeneca, Novartis, Pfizer, Roche, and SeaGen; support for travel costs from Pfizer and Roche; has planned patents, issued or pending from EP 2951317 B1 and EP 2390370 B1; reports participation on an Advisory Board from AstraZeneca, Eisai, Lilly, Novartis, Pantarhei Bioscience, Pfizer, Pierre Fabre, Roche, and SeaGen; medical writing support from Roche. WJ reports grants and personal fees from Sanofi-Aventis, Novartis, Lilly, Pfizer, Roche, Chugai, AstraZeneca, MSD, and Daiichi Sankyo, during the conduct of the study. ES reports consulting fees from Roche and AstraZeneca; receives payment or honoraria from Roche, Pfizer, Novartis, and Astra Zeneca; support for travel costs from Roche, Pfizer, and Daiko. SSe reports reimbursement of travel costs from Novartis and personal fees from Roche, Mundipharma, and Amgen, outside the submitted work. MU reports personal fees and non-financial support from Abbvie, Amgen, AstraZeneca, Celgene, Daiichi Sankyo, Eisai, Lilly Int, MSD Merck, Mundipharma, Myriad Genetics, Odonate, Pfizer, Roche Pharma, Sanofi Aventis Deutschland, TEVA Pharmaceuticals, Novartis, and Clovis Oncology; personal fees from Bristol Myers Squibb, Lilly Deutschland, PUMA Biotechnology, Pierre Fabre, Seattle Genetics, outside the submitted work. SL reports grants or fundings from Amgen, AstraZeneca, Celgene, Novartis, Immunomedics, Pfizer, Roche, DSI Trevor, Vifor, Abbvie, Cepheid, Seagen, and VM Scope; receives payment or honoraria from AstraZeneca, Roche, Novartis, Pfizer, Prime/Medscape, Puma, and Samsung; has planned patents, issued or pending from EP14153692.0; reports participation on an Advisory Board from Amgen, AstraZeneca, DSI, Roche, Merck, Pfizer, BMS, SeaGen, Eirgenix, Celgene, Abbvie, Lilly, GlaxoSmithKline, and Pierre Fabre; is part of a leadership or fiduciary role from BIG. All other authors report no conflicts of interests., (Copyright © 2021 Elsevier Ltd. All rights reserved.)