Reduced Enpp1 activity increased the % calcified area in double mutant mice when compared to Npt2a-/- mice (Fig 4A), while the size of the calcium phosphate deposits was not affected. Similarly, intraperitoneal sodium PPi treatment reduced % calcified area, while calcification size was unchanged. Although further studies are required to define cause and effect, these data suggest that PPi inhibits nucleation (Figs 2A and 4A), which is different from the effect of Opn reported by us , that predominantly decreases mineralization size, consistent with the known role of Opn in calcium phosphate crystal growth. Interventions that increase both PPi and Opn would therefore be predicted to be additive.