The MPN PROGRESSion Registry is a multi-year, observational research study designed to improve understanding of myeloproliferative neoplasms (MPNs)-a group of rare, chronic blood cancers that include polycythemia vera (PV), essential thrombocythemia (ET), primary myelofibrosis (MF), pre-fibrotic primary myelofibrosis (pre-PMF), secondary myelofibrosis, myeloproliferative neoplasm-unclassifiable (MPN-U), MPN in accelerated phase (MPN-AP), and MPN in blast phase (MPN-BP), post-MPN Acute Myeloid Leukemia (AML), and MDS/MPN overlap syndrome as defined above per WHO 2022 criteria, including patients originally diagnosed with one of these conditions but who have received one or more SCTs and/or BMTs . These conditions are characterized by abnormal blood cell production in the bone marrow and may lead to complications such as blood clots, bleeding, bone marrow fibrosis, and, in some cases, progression to acute leukemia. The central hypothesis of the registry is that collecting and analyzing real-world, longitudinal data-including electronic health records (EHRs), laboratory values, treatments, and patient-reported outcomes (PROs)-from a diverse population of people living with MPNs will help identify patterns and predictors of disease progression, treatment response, quality of life, and long-term outcomes. These insights are intended to guide future research, inform clinical guidelines, and support improvements in patient care. The registry is non-interventional and observational; participants do not receive investigational treatments, and all medical care continues under the supervision of their own physicians. Data collection includes EHRs, PRO surveys, patient-reported symptom and lab tracking, insurance claims, and, in the future, may include linkages with other relevant disease registries and datasets. Potential collaborations under consideration include those with the European LeukemiaNet (ELN) MPN Registry, the Mayo Clinic MPN Database, the Center for International Blood and Marrow Transplant Research (CIBMTR), the SEER Program, Harmony Alliance Foundation, and the National Cancer Database (NCDB). The registry emphasizes the patient voice, incorporating lived experiences related to hallmark MPN symptoms such as fatigue, pruritus (itching), bone pain, night sweats, and social and emotional impacts. Participants will be followed for at least five years, with many enrolled for ten years or longer, to capture the natural history of disease and long-term outcomes. PRO surveys will be completed approximately every six months, and EHR data will be regularly reviewed to track changes in clinical status, treatment, and disease evolution. Statistical analyses will use descriptive and inferential methods to examine clinical characteristics, symptom burden, disease trajectories, and patient-centered outcomes. Planned subgroup analyses may compare differences across diagnoses, treatment approaches, demographics, or genomic factors. Analytic plans will be finalized during the course of the study and may evolve in response to emerging scientific questions. The registry is open to adults (18 years or older) living in the United States who have been diagnosed with any of the included MPN subtypes and are willing to share health information and complete study surveys. Individuals currently enrolled in interventional clinical trials or unable to provide informed consent may be excluded. Participation is voluntary, and participants may withdraw from the study at any time without affecting their medical care. Privacy and data security are core priorities. Participant data will be securely stored and managed in accordance with all applicable privacy laws and research regulations. No identifiable information will be shared with external parties without appropriate authorization. Oversight is provided by a Steering Committee and a Patient Engagement Advisory Committee (PEAC), ensuring rigorous scientific, ethical, and patient-centered governance. The registry is sponsored by the MPN Research Foundation, a nonprofit organization advancing research and patient advocacy in myeloproliferative neoplasms (MPNs). Participants can contact the registry team at any time with questions and will receive periodic updates on study findings. This study aims to address critical gaps in understanding the real-world experiences of people with MPNs-such as symptom burden over time, risk factors for progression, and how different treatments impact patient outcomes. Findings may inform clinical trial design, support biomarker discovery, and contribute to the development of updated treatment recommendations. The registry is committed to including participants from diverse backgrounds and clinical settings to ensure findings are broadly applicable across the MPN community. Summary results will be shared through scientific publications, presentations, and other dissemination efforts to advance MPN research and care globally.