ABCC6 and Pseudoxanthoma elasticum: In the present study we have found that even though biallelic ABCC6 mutations do decrease plasma PPi level, the highly variable plasma PPi concentrations measured in our PXE cohort cannot be attributed to the specific type of ABCC6 mutations these individuals harbor, even though ABCC6 protein has a key role in the metabolic pathway responsible for approximately 60% of circulating extracellular PPi [2] (see Figure 4A).