Acts as transcription regulator that binds target promoter DNA and bends the DNA (PubMed:8636240, PubMed:24153254, PubMed:19328208). Binds to the sequences 5'-AACAAT-'3 or 5'-AACAAAG-3' (PubMed:8636240). Modulates transcriptional regulation via WNT3A. Inhibits Wnt signaling. Promotes degradation of activated CTNNB1. Plays a key role in the regulation of embryonic development (PubMed:11973269, PubMed:17655922, PubMed:24153254). Required for normal development of the definitive gut endoderm (PubMed:11973269). Required for normal looping of the embryonic heart tube. Plays an important role in embryonic and postnatal vascular development, including development of arteries (PubMed:24153254). Plays an important role in postnatal angiogenesis, where it is functionally redundant with SOX18 (PubMed:16895970). Required for the generation and maintenance of fetal hematopoietic stem cells, and for fetal hematopoiesis (PubMed:17655922). Probable transcriptional activator in the premeiotic germ cells. Has no DNA-binding activity, and does not function as transcriptional activator. Interacts with CTNNB1, LEF1 and TCF4. Detected in lung and testis (PubMed:8636240). Detected in endothelial cells around small and large arteries in newborns and adults, but is barely detectable in veins (at protein level) (PubMed:24153254). Detected in lung and testis (PubMed:8636240). Detected in the extraembryonic region of the visceral endoderm of pre-streak and early-streak embryos. Detected in the extraembryonic region of the visceral endoderm and in the definitive endoderm at 7.5 dpc. By the seven to eight somite stage, detected in the posterior endoderm, mainly in the endoderm of the midgut and hindgut invagination. Expressed in spermatogonia. The expression clearly declines from the early pachytene spermatocyte stage onward. In contrast, expression of isoform 2 (T-SOX17) begins at the pachytene spermatocyte stage and is highly accumulated in round spermatids (PubMed:11973269). Detected in arterial endothelium in embryos and yolk sac at 10.5 dpc (PubMed:24153254). The 9aaTAD motif is a transactivation domain present in a large number of yeast and animal transcription factors. Embryonic lethal due to defects in the development of the definite gut endoderm, aberrant heart looping and severe defects in vascular development and remodeling, including a lack of artery formation. Embryos die at about 10.5 dpc (PubMed:11973269, PubMed:17655922, PubMed:24153254). Besides, mutant embryos display a severe defect in fetal hematopoiesis (PubMed:17655922).