BCOR (BCL6 corepressor) functions as a transcriptional corepressor that plays critical roles in hematopoiesis, development, and disease pathogenesis 1. As a component of polycomb repressive complex 1.1 (PRC1.1), BCOR regulates gene expression through chrX modifications and histone deacetylase activities 2. In hematopoietic contexts, BCOR acts as a tumor suppressor, with disruptive mutations leading to loss of full-length protein function 1. These mutations are frequently observed in acute myeloid leukemia, where they define distinct genetic subtypes associated with secondary AML development and worse clinical outcomes 3. BCOR mutations also occur in aplastic anemia, where they correlate with better immunosuppressive therapy response and improved survival 4. Beyond hematologic malignancies, BCOR alterations drive various sarcomas, including CNS high-grade neuroepithelial tumors and BCOR-CCNB3 fusion sarcomas, which show distinct morphologic features and clinical behaviors 56. In T cell biology, BCOR suppresses stemness programs in exhausted CD8+ T cells, with its loss enhancing precursor-exhausted T cell proliferation and functionality 7. The protein's diverse roles across multiple biological systems underscore its importance as a chrX regulator controlling cell fate decisions.