Transcription factor involved in chondrocytes differentiation and cartilage formation (PubMed:1396566, PubMed:11702786, PubMed:8078769, PubMed:15529345, PubMed:14993235). Specifically binds the 5'-AACAAT-3' DNA motif present in enhancers and super-enhancers and promotes expression of genes important for chondrogenesis, including cartilage matrix protein-coding genes, such as COL2A1 and AGC1 (PubMed:9755172, PubMed:11702786, PubMed:26150426). Required for overt chondrogenesis when condensed prechondrocytes differentiate into early stage chondrocytes: SOX5 and SOX6 cooperatively bind with SOX9 on active enhancers and super-enhancers associated with cartilage-specific genes, and thereby potentiate SOX9's ability to transactivate (PubMed:11702786, PubMed:15529345, PubMed:14993235, PubMed:26150426). Not involved in precartilaginous condensation, the first step in chondrogenesis, during which skeletal progenitors differentiate into prechondrocytes (PubMed:14993235). Together with SOX6, required to form and maintain a pool of highly proliferating chondroblasts between epiphyses and metaphyses, to form columnar chondroblasts, delay chondrocyte prehypertrophy but promote hypertrophy, and to delay terminal differentiation of chondrocytes on contact with ossification fronts (PubMed:14993235). Binds to the proximal promoter region of the myelin protein MPZ gene (PubMed:26525805). Forms homodimers and heterodimers with SOX6. Expressed in the embryo and in adult testis. Expressed in chondrocytes and, to a lesser extent, in brain. Testis-specific. Expression is transiently increased during brown adipocyte differentiation. Expressed during spermatogenesis. Expression is dependent on SOX9. Newborn mice display mild skeletal abnormalities (PubMed:11702786). Mice lacking both Sox5 and Sox6 develop a severe chondrodysplasia characterized by the virtual absence of cartilage: chondrogenic cells are largely arrested at the stage of chondrogenic mesenchymal condensations (PubMed:11702786). Embryos lacking Sox5 (homozygous knockout) and heterozygous for Sox6 live until birth and show severe growth plate chondrocyte defects (PubMed:14993235). Embryos lacking Sox6 (homozygous knockout) and heterozygous for Sox5 live until birth and show severe growth plate chondrocyte defects (PubMed:14993235).