ENPP1 and pseudoxanthoma elasticum (inherited or acquired): PXE occurs due to biallelic null mutations primarily in Abcc6 (ATP binding cassette sub family C, member 6), an ATP transporter produced in the liver responsible for moving ATP from the intracellular space to the extracellular environment where it is cleaved by ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) to produce AMP and PPi, a potent inhibitor of nanohydroxyapatite formation.