Transcriptional regulator with a major role in neural stem cell commitment and corticogenesis as well as in lymphoid cell development and lymphoid tissue organogenesis (PubMed:25527292, PubMed:20818394, PubMed:11850626, PubMed:18195075, PubMed:15078895, PubMed:25915732). Binds to GC-rich DNA sequences in the proximity of transcription start sites and may alter chromatin structure, modifying access of transcription factors to DNA (PubMed:25527292, PubMed:31207603, PubMed:31207604). During cortical development, controls the neural stem cell pool by inhibiting the switch from proliferative to differentiating progenitors. Beyond progenitor cells, promotes neurite outgrowth in newborn neurons migrating to reach the cortical plate. May activate or repress critical genes for neural stem cell fate such as SOX2, EOMES and ROBO2 (PubMed:25527292). Plays an essential role in the development of lymphoid tissue-inducer (LTi) cells, a subset necessary for the formation of secondary lymphoid organs: peripheral lymph nodes and Peyer's patches (PubMed:20818394). Acts as a developmental checkpoint and regulates thymocyte positive selection toward T cell lineage commitment (PubMed:11850626, PubMed:18195075). Required for the development of various T cell subsets, including CD4-positive helper T cells, CD8-positive cytotoxic T cells, regulatory T cells and CD1D-dependent natural killer T (NKT) cells (PubMed:18195075, PubMed:15078895). Required for the differentiation of common lymphoid progenitors (CMP) to innate lymphoid cells (ILC). May regulate the NOTCH-mediated gene program, promoting differentiation of the ILC lineage (PubMed:25915732). Required at the progenitor phase of NK cell development in the bone marrow to specify NK cell lineage commitment (PubMed:20818394). Upon chronic antigen stimulation, diverts T cell development by promoting the generation of exhaustive T cells, while suppressing effector and memory T cell programming. May regulate the expression of genes encoding inhibitory receptors such as PDCD1 and induce the exhaustion program, to prevent the overstimulation of T cells and activation-induced cell death (PubMed:31207603, PubMed:31207604). Interacts with HBO1 complex composed at least of KAT7/HBO1, ING4, MEAF6, and JADE2; this complex is involved in histone acetylation. Interacts with DNMT1, LEO1, PAF1, SAP130 and SIN3A; these interactors regulate chromatin remodeling. Interacts with an array of proteins involved in RNA processing and translation and DNA replication. Expressed in neurons of the subventricular zone (at protein level) (PubMed:25527292). Expressed in distinct subpopulations of thymocytes undergoing positive selection: double CD4-positive CD8-positive (DP) cells, CD4-positive CD8-low transitional cells and in single CD4-positive and CD8-positive cells (at protein level) (PubMed:11850626, PubMed:15078895). Expressed in ILC progenitors and mature ILC subsets: ILC1, ILC2 and ILC3 (at protein level) (PubMed:25915732). Expressed in lymphoid tissue-inducer cells and bone marrow NK cell subsets (PubMed:20818394). Abundant in thymus, liver and brain. Also detected in small intestine, spleen, stomach and testis (PubMed:11850626). Highly expressed in tumor-infiltrating CD8-positive T cells (at protein level) (PubMed:31207604). In the developing brain, expressed at embryonic day 9.5 dpc in neuroepithelium, displaying a rostral-high/ caudal-low and lateral-high/medial-low expression pattern. Abundant at 15.5 dpc in progenitors of the ventricular zone and differentiated neurons in the cortical plate. The lateral-medial gradient spread further in all cells of the ventricular zone of the lateral cortex by 18.5 dpc (at protein level). Transiently up-regulated during key developmental transition of immune cell subsets, likely marking a developmental checkpoint. Up-regulated during beta and positive selection of developing thymocytes, upon activation of pre-T cell receptor or T cell receptor in a calcineurin-dependent manner (PubMed:11850626, PubMed:15078895). Low expression is detected in precursor bone marrow NK cells, then is up-regulated in immature and mature bone marrow NK cells and later down-regulated in splenic mature NK cells (PubMed:20818394). (Microbial infection) Up-regulated in LCMV-specific CD8-positive T cells. Expressed at high levels in exhausted T cells during chronic infection. The HMG box is critical for TOX-dependent CD4-positive T cell lineage commitment. Mutant mice lack lymph nodes. The development of Peyer's patches is compromised, detectable only in some mutants. Peyer's patches are much smaller in size less abundant when compared to wild-type littermates. T cell lymphopenia is a hallmark phenotype of TOX-deficient mice. Belongs to the high motility group (HMG) box superfamily.