FGF23 and osteomalacia: Different molecular mechanisms – overproduction of FGF-23 by tumors responsible for oncogenic osteomalacia, generation of an FGF-23 mutant that is resistant to cleavage by enzymes, and impaired FGF-23 degradation due to the reduction or loss of PHEX – can lead to FGF-23-stimulating activity and the consequent waste of urinary phosphate (13).