Abstract Background: Genomic expression profiles have implications for the personalized treatment of breast cancer beyond clinical and pathological features by enabling the classification of breast cancers into molecular subtypes and providing prognostic information about the metastatic potential of tumors. However, full genome expression data should be combined with comprehensive clinical information to precisely stratify tumors into clinically actionable subgroups. The FLEX Registry aims to aggregate a large, real-world dataset, which will enable the discovery of novel genomic profiles to improve precision in the management of breast cancer, particularly in underrepresented patient subsets in traditional clinical trials. Trial Design: The FLEX Registry (NCT03053193) is a multi-center, prospective, observational trial for patients with stage I-III breast cancer whose primary tumor is analyzed by MammaPrint, with or without BluePrint.The primary objective of FLEX is to create a large scale, population-based registry that links comprehensive clinical data with full genome expression data to elucidate new prognostic and/or predictive gene associations in a real-world setting. The FLEX Registry employs a shared study infrastructure to develop and investigate hypotheses for targeted subset analyses and/or clinical trials based on full genome expression data. The adaptable protocol is designed to be amended with the inclusion of additional targeted sub-studies. Patients enrolled in the initial study are eligible for inclusion in sub-studies for which they meet all eligibility criteria and additional consent is not required. Data will be collected on patients from diagnosis through10 years of follow-up and any necessary additional clinical data will be collected as specified in the appendix protocols. The target enrollment of FLEX is a minimum of 10,000 patients; over 5,000 patients have enrolled sinceApril 2017 at more than 85 sites, including eight National Cancer Institute-designated comprehensive cancer centers. The FLEX collaborative platform allows participating investigators the opportunity to author their own sub-study protocols, as approved by the FLEX Review Committee. Sub-study research categories include: Age and Breast Cancer, Optimizing Therapy Strategies, Breast Cancer and MetabolicSyndrome, ctDNA and Liquid Biopsy, Genomics and Subtypes, Social and Ancestry, and Neoadjuvant Therapy and Surgery. To date, twenty-five investigator-initiated sub-studies have been approved.Trial contact information: NCT03053193FLEX@agendia.com Citation Format: Laura Lee, Sami Diab, Julie Barone, Jennifer A Crozier, Margret Chen, Rakshanda L Rahman, Robert Maganini, Douglas Marks, Amy M Truitt, Lisa Blumencranz, Erin Yoder, Sarah Untch, William Audeh, Bastiaan van der Baan, FLEX Investigators Group. The FLEX real-world data platform explores new gene expression profiles and investigator-initiated protocols in early stage breast cancer [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr OT-12-01.