Injury and/or arthritis causes damage to cartilage in joints. Cartilage is a highly differentiated and specialized cell type that has almost no capacity for healing or replacing itself when seriously injured. In this study, researchers transferred a gene that stimulates the synthesis of cartilage components into cells; these cells will then be grafted into a damaged joint. The study was done on horses; horses provide an excellent model because they suffer similar clinical problems as humans. In addition, horses have large joints, cartilage thickness and underlying bone, all of which closely approximate those of humans.