Enpp1 expression is positively regulated by phosphate in osteoblast cultures , and therefore we expected that expression is likewise increased in Npt2a-/- mice to explain the increased urine PPi levels. Instead, we found that Enpp1 expression is unchanged, possibly as a result of reduced Pi sensing in the absence of Npt2a. Furthermore, Ank expression was decreased and Tnsalp was increased, all predicted to reduce local PPi production. These findings suggest that PPi may be generated outside of the kidneys contrary to previous reports [25, 26], and elevate urine PPi despite unchanged or decreased local gene expression for Enpp1 and Ank, respectively. Consistent with this hypothesis is our finding that global reduction of Enpp1 activity in Enpp1asj/asj mutant mice decreased urine PPi levels and that intraperitoneal injection of sodium pyrophosphate increased urine PPi levels (Fig 4G). Alternatively, PPi production may be regulated locally by increased renal activities of Enpp1 and Ank on a post-transcriptional level.