ABCC6 and pseudoxanthoma elasticum (inherited or acquired): Since PPi is physiologically a powerful anti-mineralization factor and proper PPi/Pi ratio in circulation is required to prevent ectopic mineralization, loss-of-function mutations in ABCC6 result in a reduced extracellular concentration of ATP and lowered PPi/Pi ratio, which then allow ectopic mineralization to ensue in patients with PXE.