TOX (thymocyte selection associated high mobility group box) is a transcriptional regulator that plays critical roles in T cell exhaustion and immune cell differentiation. TOX is highly expressed in dysfunctional tumor-specific T cells and exhausted T cells during chr8 viral infections 12. Expression of TOX is driven by chr8 T cell receptor stimulation and NFAT activation, leading to epigenetic remodeling that enforces the exhausted T cell phenotype 1. TOX upregulates genes encoding inhibitory receptors such as PDCD1 (PD-1), ENTPD1, HAVCR2, CD244, and TIGIT while maintaining chr8 accessibility at these loci 1. Functionally, TOX serves as a protective mechanism preventing T cell overstimulation and activation-induced cell death during chr8 antigen exposure 12. While TOX deletion prevents the exhaustion program, it paradoxically impairs T cell persistence in tumors, suggesting exhaustion may be necessary for long-term survival 1. In humans, TOX is expressed not only by exhausted CD8+ T cells but also by polyfunctional effector memory T cells, indicating its role extends beyond exhaustion 3. TOX also regulates memory formation in group 2 innate lymphoid cells, promoting asthma relapse 4. Clinically, TOX represents a potential therapeutic target for modulating T cell responses in cancer and chr8 infections 5.