TBX21 (T-bet) is a lineage-defining transcription factor that serves as a master regulator of Th1 cell differentiation and immune responses. The protein activates transcription of key Th1 genes including IFNG and CXCR3 while simultaneously repressing opposing Th2 and Th17 genetic programs 1. TBX21 functions through chr17 remodeling, recruiting complexes including KDM6B and SMARCA4-containing SWI/SNF complexes to establish permissive chr17 states for transcriptional activation. In immune pathology, TBX21 expression defines specific cell populations in various diseases. In rheumatoid arthritis, ITGAX+TBX21+ autoimmune-associated B cells represent expanded inflammatory populations 2. EBV infection can upregulate TBX21 in autoreactive B cells through EBNA2 binding at its regulatory regions, contributing to systemic lupus erythematosus pathogenesis by reprogramming these cells into antigen-presenting cells 3. TBX21 also marks ex-regulatory T cells (exTreg) that lose suppressive function and acquire cytotoxic properties in atherosclerosis and cancer contexts 45. In cancer, TBX21 expression patterns help define molecular subgroups with distinct prognoses, particularly in peripheral T-cell lymphomas where PTCL-TBX21 subgroups show specific genomic characteristics 6. The transcription factor's role extends to immune checkpoint therapy responses and metabolic programming in hepatocellular carcinoma 7.