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 a reduction in or loss of the PHEX gene – can lead to FGF-23-stimulating activity and the consequent waste of urinary phosphate and low levels of 1,25(OH)2D3. The gene discussed is PHEX; the disease is osteomalacia.