ABCC6 and Pseudoxanthoma elasticum: Mutations in either ABCC6 (ATP binding cassette subfamily C member)or ENPP1 (ectonucleotide pyrophosphatase 1)can result in the severe phenotype of GACI, often culminating in mortality within the first year of life [249]. Mutations in ENPP1 can lead to typical pseudoxanthomatous skin lesions and angioid streaks of the retina in infants with GACI who have survived infancy; nevertheless, the later emergence of the classic PXE phenotype without GACI has only been seen in patients with mutations in ABCC6.