ABCC6 and pseudoxanthoma elasticum (inherited or acquired): By contrast, dysfunction of the ATP-binding cassette (ABC)-transporter ABCC6 (coding for the transmembrane protein MRP6 highly expressed in liver and kidney) causes pseudoxanthoma elasticum (PXE), a rare disease characterized by mineralization and degeneration of elastic fibers within soft connective tissues, thus causing skin laxity, cardiovascular complications, and visual impairment in a setting of normal levels of circulating calcium and phosphate and without bone abnormalities (see ahead for further details; Quaglino et al., 2011).