VDR and Dowling-Degos disease 1: However, there is increasing evidence that DDD is a genetic disorder [12], with reports of predisposition in association with certain genes, such as Taq I polymorphism of vitamin D receptor (VDR) gene [2,5,13,14], polymorphism of metalloproteinase-3 (MMP3) gene [15-17], mutations in collagen IX genes (COL9A2 and COL9A3) [18-22], Sp1 polymorphism of collagen I gene (COL1A1) [23,24], polymorphism of cartilage intermediate layer protein (CILP) [25] as well as aggrecan gene polymorphism [26].