Multiple epiphyseal dysplasia (MED) may be caused by mutations of genes encoding collagen IX, namely COL9A1 (encoding α1 chain), COL9A2 (α2 chain), and COL9A3 (α3 chain), but such mutations are not the main cause of MED [55]. The gene discussed is COL9A3; the disease is multiple epiphyseal dysplasia.