ABCC6 and arterial calcification, generalized, of infancy, 1: Biallelic inactivating variants in ENPP1 (ectonucleotide pyrophosphatase/phosphodiesterase 1) or ABCC6 (ATP binding cassette subfamily C member 6) account for ~75% and 9% of GACI cases, respectively.(4, 7) ENPP1 is an integral transmembrane protein that hydrolyzes extracellular adenosine triphosphate (ATP) to inorganic pyrophosphate (PPi) and adenosine monophosphate (AMP).