SPP1 and Pseudoxanthoma elasticum: By searching the literature, we discovered 13 modifier genes with a variety of functions, including antioxidant and cellular stress genes such as catalase and SOD2. Genes involved in mineralization and calcium metabolism pathways, such as XYLT1/2, MGP, and MMP2, genes involved in the breakdown of ATP, such as ENPP1, and genes involved in angiogenesis, such as VEGFA. Another modifier gene, SPP1, may also be associated with PXE.