Transcription factor that plays a key role in chondrocytes differentiation and skeletal development. Specifically binds the 5'-ACAAAG-3' DNA motif present in enhancers and super-enhancers and promotes expression of genes important for chondrogenesis, including COL2A1. Plays a central role in successive steps of chondrocyte differentiation. Absolutely required for precartilaginous condensation, the first step in chondrogenesis during which skeletal progenitors differentiate into prechondrocytes. Together with SOX5 and SOX6, required for overt chondrogenesis when condensed prechondrocytes differentiate into early stage chondrocytes, the second step in chondrogenesis. Later, required to direct hypertrophic maturation and block osteoblast differentiation of growth plate chondrocytes: maintains chondrocyte columnar proliferation, delays prehypertrophy and then prevents osteoblastic differentiation of chondrocytes. Also required for chondrocyte hypertrophy, both indirectly, by keeping the lineage fate of chondrocytes, and directly, by remaining present in upper hypertrophic cells. Low lipid levels are the main nutritional determinant for chondrogenic commitment of skeletal progenitor cells: when lipids levels are low, FOXO transcription factors promote expression of SOX9, which induces chondrogenic commitment and suppresses fatty acid oxidation. In addition to cartilage development, also acts as a regulator of proliferation and differentiation in epithelial stem/progenitor cells (By similarity). Unlikely to play a role in sex determination but may function during testicular and ovarian differentiation (PubMed:18692165, PubMed:18816826). Interacts with the sumoylation factors ube2i/ubc9 and sumo1. Restricted to the nucleus of Sertoli-like cells in the testis, but localizes to the cytoplasm of previtellogenic oocytes in the ovary before being translocated into the nucleus of vitellogenic oocytes. Expressed in both male and female gonads from after metamorphosis through to adult stages. In the testis, expression is restricted to the supporting Sertoli-like cells. Conversely in the ovary, expression is localized to primary oocytes (at protein level). In developing limbs, expressed before chrondrocytes form (stage 52 tadpoles) and throughout the cartilaginous anlagen until stage 56, after which expression ceases in the enlarged cells of the diaphysis. At later stages, expression continues in the chondrocytes of the epiphysis and metaphysis, and weak expression is seen in most of the diaphysis. The 9aaTAD motif is a transactivation domain present in a large number of yeast and animal transcription factors. Sumoylated. Lys-370 is the major site of sumoylation, although sumoylation at Lys-61 also occurs. Sumoylation plays a key role in regulating formation of the neural crest and otic placode (By similarity).