Myeloproliferative Neoplasm (MPN)

This phase 1 trial will investigate the safety and effectiveness of Thiotepa, Busulfan, and Fludarabine (TBF) conditioning regimen with post-transplant cyclophosphamide (PTCy) in HLA-matched related or unrelated donor allogeneic stem cell transplantation (alloSCT).

The goal of this observational study is to learn how the immune system responds to myeloproliferative neoplasms (MPN) in adults with known or suspected MPN. Researchers also want to identify immune cells that could help develop future immune-based treatments for MPN. The main questions it aims to answer are: * Are there T cells in the bone marrow that can recognize MPN cells? * Which targets (antigens) on MPN cells are recognized by these immune cells? * Can researchers identify T cell receptors (TCRs) that may be used in future TCR-based therapies? Researchers will study samples from adults with polycythemia vera (PV), essential thrombocythemia (ET), primary myelofibrosis (PMF), post-PMF, or unclassifiable MPN. Samples from healthy donors without blood disorders will also be included for comparison. By providing blood and bone marrow samples, participants will: * Allow researchers to study immune cells and genetic material from these samples * Allow researchers to perform laboratory tests to study how immune cells recognize MPN cells Researchers will use laboratory methods such as genetic sequencing, cell analysis, and cell culture experiments to better understand immune responses in MPN. Participants will not receive direct medical benefit from this study. The results may help researchers better understand MPN and support the future development of immune-based therapies.

The purpose of this clinical trial is to compare drug combinations to learn which drugs work best to prevent graft-versus-host-disease (GVHD) in people who have received a stem cell transplant. The source of stem cells is from someone who is not related and has a different blood cell type than the study participant. The researchers will compare the new drug combination to a standard drug combination. They will also learn about the safety of each drug combination. Participants will: * Receive the standard or new drug combination after transplant * Visit the doctor's office for check-ups and tests after transplant that are routine for most transplant patients * Take surveys about physical and emotional well-being * Give blood and stool samples.