SOX5 (SRY-related HMG box) is a 75 - 80 kDa Group D member of the SOX family of DNA-binding proteins. It is expressed in fetal chondrocytes and skeletal muscle, with a short form appearing in spermatids. SOX5 form homodimers and heterodimerizes with SOX6. Human SOX5 is 763 amino acids (aa) in length and contains two coiled-coil regions (aa 193 - 274 and 448 - 515), plus one Q/glutamine (aa 234 - 272) and one HMG box (aa 556 - 624). HMG domains participate in DNA-binding and protein-protein interaction. There are two alternate start sites at Met14 and Met417 (which creates a short/testis form), plus one variant with a three aa substitution for aa 389 - 763, another variant with a two aa substitution for aa 1 - 388, and a third variant with a one aa substitution for aa 534 - 763. Over aa 417 - 763, human SOX5 is 97% aa identical to mouse SOX5.