ABCC6 and Pseudoxanthoma elasticum: Xanthine, another metabolite in purine metabolism, was strongly diminished in PXE fibroblasts (Figure 7b; patient/control ratio:0.1, p<0.0001) and slightly depleted in ABCC6-silenced cells (Figure 7b; siABCC6/siNK ratio:0.6, p = 0.1).