PAX5 is a B-cell lineage-specific transcription factor essential for B lymphocyte development and differentiation 1. As a master regulator of B-cell commitment, PAX5 functions as a metabolic gatekeeper by transcriptionally repressing glucose uptake and ATP synthesis, maintaining energy deprivation that prevents malignant transformation 1. This metabolic restriction involves direct regulation of NR3C1, TXNIP, and CNR2, with PAX5 loss causing >25-fold increases in glucose uptake and ATP in pre-B cells 1. Clinically, PAX5 lesions occur in >80% of pre-B acute lymphoblastic leukemias, highlighting its tumor-suppressive role 1. In chr9 lymphocytic leukemia, enhancer mutations on chromosome 9 reduce PAX5 expression and correlate with aggressive disease 2. STAG2 deletion impairs PAX5 transcription, reducing B-cell lineage commitment 3. Beyond hematologic malignancies, PAX5 is aberrantly expressed in 65% of Merkel cell carcinomas, though full-length PAX5 alone cannot induce B-cell differentiation without cofactors 4. PAX5 promoter methylation in gastric cancer patients associates with reduced 3-year survival 5. Additionally, PAX5 activates LINE1 retrotransposons to promote cellular senescence through cGAS-STING signaling, with SIRT6 providing suppressive regulation 6. PAX5 thus functions across diverse biological contexts as both a developmental regulator and metabolic/senescence checkpoint controller.