Figure 1.

Proposed pathogenesis of renal phosphate wasting. Mesenchymal tumors produce renal phosphate wasting by overproduction of FGF23. FGF23 levels can also be increased by mutations in Phex, a protease that cleaves and inactivates the molecule, or by mutations at key arginine residues that render FGF23 resistant to cleavage by Phex. FGF23 excess causes phosphate wasting either directly or by inducing another phosphaturic factor.