Transcription factor that binds to DNA at the consensus sequence 5'-AACAAT-3' (PubMed:9524265, PubMed:9421502). Binds to the proximal promoter region of the myelin protein MPZ gene, and may thereby be involved in the differentiation of oligodendroglia in the developing spinal tube (PubMed:26525805). Binds to the gene promoter of MBP and acts as a transcriptional repressor (PubMed:26525805). Binds to and modifies the activity of TCF7/TCF1, thereby inhibiting transcription and modulates normal gamma-delta T-cell development and differentiation of IL17A expressing gamma-delta T-cells (PubMed:17218525, PubMed:23562159, PubMed:30413363). Required for the differentiation of Vgamma2-positive gamma-delta T-cells, a subset of IL17A expressing gamma-delta T-cells (PubMed:23562159). Regulates expression of BLK in the differentiation of IL17A expressing gamma-delta T-cells (PubMed:23562159). Promotes brown adipocyte differentiation (PubMed:27923061). Inhibitor of WNT signaling (By similarity). Homodimer; homodimerization reduces DNA binding efficiency (By similarity). Interacts with isoform 2 of TCF7/TCF1 (via N-terminus); inhibits WNT-mediated transcriptional activity (PubMed:17218525). Interacts with HHEX (via N-terminus); abolishes the SOX13-mediated inhibition of WNT-mediated transcriptional activity via competitive inhibition of the SOX13-TCF7 complex (PubMed:20028982). Expressed in beta-cells in pancreatic islets of Langerhans (at protein level) (PubMed:10871192). Expressed in a subset of invariant natural killer (iNK) T-cells (at protein level) (PubMed:30413363). Expressed in oligodendroglial cells in the spinal cord (at protein level) (PubMed:26525805). In the hematopoietic system, expressed in gamma-delta T-cell lineage, specifically in immature Vgamma2-positive gamma-delta thymocytes, with expression decreasing upon maturation (PubMed:17218525, PubMed:23562159, PubMed:30413363). Expressed in DN1d cells, a subset of DN1 (TCR, CD4 and CD8 negative, also known as TN1) thymocyte progenitors, and in DN2 (also known as TN2) thymocyte progenitors (PubMed:17218525, PubMed:30413363). Expressed in the ovary and kidney (PubMed:9524265). Expressed in the peripheral lymphoid organs (PubMed:17218525). Expressed in the visceral mesoderm of the yolk sac from 9.5 dpc to 15.5 dpc (at protein level) (PubMed:16835393, PubMed:30413363). Expressed in the sclerotome of developing somites and the derivative vertebrae and ribs between 9.5 dpc and 12.5 dpc (PubMed:16835393). Expressed in the rostral perichondrial area and mesenchyme of the limb bud in the prospective arm region and hand plate at 10.5 dpc, proximal mesenchyme expression in the forelimb then decreases between 11.5 dpc and 12.5 dpc until expression is consolidated to the arm regions of the developing limb at 12.5 dpc and forelimb digits at 13.5 dpc (PubMed:16835393). Expressed at variable levels in the prospective forebrain to the hindbrain and all neural tube cell layers from 9.5 dpc to postnatal day 6 (P6) (PubMed:16835393, PubMed:26525805). Expressed in immature oligodendrocyte precursor cells and astrocytes at 15.5 dpc, additionally expressed in precursor and differentiating oligodendrocytes in the neural tube from 18.5 dpc to P6 (PubMed:26525805). Highly expressed in embryonic arterial walls at 13.5 dpc (PubMed:9421502). Low levels are found in the inner ear at 13.5 dpc and in some cells in the thymus at 16.5 dpc (PubMed:9421502). Initially expressed in the hair follicles of the whisker pad vibrissae at 14.5 dpc, expression continues in the hair placode and in hair germ and peg during hair follicle morphogenesis to 18 dpc (PubMed:9421502, PubMed:30638933). Expressed in the newborn hair follicle epithelial sheath, this expression weakens at P15, reexpressed in the hair follicle bulge and secondary germ regions at P22, with expression continuing in the root sheath of the hair follicle at P28 (PubMed:30638933). Expressed in trophoblast giant cells of the spongiotrophoblast and labyrinth layers of the placenta at 15.5 dpc (PubMed:16835393). Expressed in DN1d cells, a subset of DN1 precursor thymocytes, from 16 dpc onwards (PubMed:30413363). Expressed in the tracheal epithelium below the vocal cord at 18 dpc (PubMed:9421502). Expression is transiently increased during brown adipocyte differentiation (PubMed:27923061). Mice show severe growth abnormalities (PubMed:17218525). Severe reduction in the number of Rorc and Vgamma2-positive mature thymocytes in fetal and adult mice caused by the loss of Blk expression in Vgamma2-positive immature thymocytes (PubMed:23562159). This results in a severe reduction of Il17a-producing Vgamma2-positive gamma-delta T-cells in the spleen, lymph nodes and dermis (PubMed:17218525, PubMed:23562159). In addition, the number of Vgamma4-positive thymocytes in the fetal thymus is transiently reduced, with levels returning to normal in neonatal and adult mice (PubMed:23562159). Proliferation of CD3, CD4, and CD8-negative (triple negative) thymocytes and gamma-delta thymocytes is increased (PubMed:17218525). Double knockout of Sox13 and Tcf7/Tcf1 shows a reduced number of DN1d cells (PubMed:30413363). No defects in oligodendrocyte precursor cells specification and in their differentiation into myelinating oligodendrocytes (PubMed:26525805). However, in Sox13 and Sox6 double knockout mice, causes a slight increase in the number of prematurely differentiated oligodendrocytes in the spinal tube compared to Sox6 knockout mice (PubMed:26525805). No defects in the integumentary system morphology and in hair development (PubMed:30638933). Several frameshifts.