Light micrographs of 10 um renal sections prepared from paraffin-embedded kidneys, obtained from mice with various genotypes fed regular chow for 10 weeks (A, upper panels: von Kossa, methylene green staining, 4X, and A, lower panels: von Kossa, hematoxylin and eosin staining, 40X); Transmission electron micrographs showing microspheres in double mutant mice on regular chow, inset with larger magnification shown to the right (B); Two weeks old Npt2a-/- pups treated with i.p. injections of vehicle or sodium pyrophosphate (160 micromole/Kg/day) for two weeks (C); Histomorphometric analysis of renal mineralization (%calcified area = 100*mineralization area/tissue area, (D); calcification size = mineralization area/number of calcifications, um2, (E), and plasma pyrophosphate levels (F) and urine pyrophosphate (U-PPi) (G) of two weeks old Npt2a-/- pups treated with i.p. injections of vehicle or sodium pyrophosphate (160 micromole/Kg/day) for two weeks, measured after overnight fast and 18–24 hrs. following the last treatment. The data represent individual animals (closed circles) with the means±SEM, p-values shown above the lines of comparisons were calculated by Student’s t-test.