Lineage-defining transcription factor which initiates Th1 lineage development from naive Th precursor cells both by activating Th1 genetic programs and by repressing the opposing Th2 and Th17 genetic programs. Activates transcription of a set of genes important for Th1 cell function, including those encoding IFN-gamma and the chemokine receptor CXCR3. Activates IFNG and CXCR3 genes in part by recruiting chromatin remodeling complexes including KDM6B, a SMARCA4-containing SWI/SNF-complex, and an H3K4me2-methyltransferase complex to their promoters and all of these complexes serve to establish a more permissive chromatin state conducive with transcriptional activation (PubMed:10761931, PubMed:17923685, PubMed:21095589). Can activate Th1 genes also via recruitment of Mediator complex and P-TEFb (composed of CDK9 and CCNT1/cyclin-T1) in the form of the super elongation complex (SEC) to super-enhancers and associated genes in activated Th1 cells (PubMed:27292648). Inhibits the Th17 cell lineage commitment by blocking RUNX1-mediated transactivation of Th17 cell-specific transcriptinal regulator RORC (PubMed:21151104). Inhibits the Th2 cell lineage commitment by suppressing the production of Th2 cytokines, such as IL-4, IL-5, and IL- 13, via repression of transcriptional regulators GATA3 and NFATC2 (PubMed:15662016, PubMed:21690296, PubMed:23616576). Protects Th1 cells from amplifying aberrant type-I IFN response in an IFN-gamma abundant microenvironment by acting as a repressor of type-I IFN transcription factors and type-I IFN- stimulated genes (PubMed:28623086). Acts as a regulator of antiviral B-cell responses; controls chronic viral infection by promoting the antiviral antibody IgG2a isotype switching and via regulation of a broad antiviral gene expression program (PubMed:27430722). Interacts with RUNX1 and RUNX3 (PubMed:21151104). Interacts with ITK (PubMed:15662016). The phosphorylated form (at Tyr-525) interacts with GATA3 (PubMed:15662016, PubMed:21690296, PubMed:23616576). Interacts with ABL1 (PubMed:21690296). Interacts with RELA (PubMed:23616576). The phosphorylated form (at Thr-302) interacts with NFATC2 (PubMed:23616576). Interacts with KDM6B (PubMed:21095589). Interacts with SMARCA4 in a KDM6B-dependent manner (PubMed:21095589). Interacts with CCTN1 and CDK9 (PubMed:27292648). Interacts with USP10 (By similarity). T-cell specific (PubMed:10761931, PubMed:11087660). Expressed in regulatory T (TReg) cells (PubMed:28607488). Induced during early Th1 cell differentiation, gradually decreasing at later stages. Phosphorylations at Ser-52, Tyr-76, Ser-224 and Ser-508 are regulated by mTORC1 (PubMed:28424242). Phosphorylation at Tyr-525 is essential for its interaction GATA3 (PubMed:15662016). Phosphorylation at Tyr-219, Tyr-265 and Tyr-304 enhances its transcriptional activator activity (PubMed:21690296). Phosphorylation at Thr-302 is required for its interaction with NFATC2 (PubMed:23616576). Ubiquitinated at Lys-313, leading to its degradation by the proteasome. Ubiquitination is essential for controlling protein stability, binding to the T-box-binding element of the IFN-gamma promoter, and for interaction with NFATC2 through induction of phosphorylation at Thr-302 (PubMed:23616576). Deubiquitinated by USP10 leading to its stabilization (By similarity).