Enpp1asj/asj mice develop renal mineralization on a “stone-forming” high phosphorus, low magnesium diet, while they develop no renal mineralization on regular chow [17, 42]. Presence of two hypomorphic asj alleles of Enpp1 blunted the increase of the urine PPi concentration of double-mutant mice when compared to Npt2a-/- mice on regular chow, albeit non-significantly (67±21, n = 4 vs. Npt2a-/- 1257±272 micromole/l, n = 19, p = 0.084, Fig 1A). Similarly, urine PPi excretion corrected for urine creatinine was decreased in double mutant mice (0.43±0.084 micromole/mg, n = 4 vs. Npt2a-/- 3.0±0.53 micromole/mg, n = 19, p = 0.044, panel A in S1 Fig). One or two hypomorphic asj alleles of Enpp1 furthermore increased the calcified area of double-mutant mice when compared to Npt2a-/- mice on regular chow in a gene dose-dependent fashion (0.3±0.07, n = 8 in Enpp1asj/+/Npt2a-/-, p = ns vs. 0.26±0.04% in Npt2a-/- and 0.69±0.15% in Enpp1asj/asj/Npt2a-/-, p<0.0001 vs. Npt2a-/-) while no mineral deposits were found in Enpp1asj/asj mice on regular chow (Fig 2A). Since increased calcified area in double mutants was due to an increase in number of calcifications, no difference was observed for mineralization size between Npt2a-/-, Enpp1asj/+/Npt2a-/-, and Enpp1asj/asj/Npt2a-/- mice (Fig 2B). Renal calcified area inversely correlated with spot urine PPi concentration (slope = -5.226e-005 ± 2.391e-005, R2 = 0.126, p = 0.036) (Fig 3A). No significant correlation was found for calcification area (Fig 3B) or when area and size were correlated with urine PPi corrected for urine creatinine (Panels C and D in S1 Fig).