Since the identification of ABCC6 as the causative gene for PXE (Bergen et al., 2000; Le Saux et al., 2000), the question of its substrate(s) has thus far eluded all interested parties, and indeed the identification of an endogenous substrate(s) for an ABC transporter is not an easy task. The gene discussed is ABCC6; the disease is Pseudoxanthoma elasticum.