ENPP1 and arterial calcification, generalized, of infancy, 1: GACI is caused by loss-of-function mutations in the ENPP1 gene, which codes for ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1), an enzyme that hydrolyzes ATP to AMP and inorganic pyrophosphate (PPi) (Ruf et al., 2005).