['Hyperostosis-hyperphosphatemia syndrome (HHS) is a rare metabolic disorder characterized by hyperphosphatemia and localized hyperostosis. HHS is caused by mutations in GALNT3, which encodes UDP-N-acetyl-alpha-D-galactosamine:polypeptide N- acetylgalactosaminyltransferase 3. Familial tumoral calcinosis (TC), characterized by ectopic calcifications and hyperphosphatemia, is caused by mutations in the GALNT3 or fibroblast growth factor 23 (FGF23) genes.', 'Our objective was to identify mutations in FGF23 or GALNT3 and determine serum FGF23 levels in an HHS patient.', 'Mutation detection in FGF23 and GALNT3 was performed by DNA sequencing, and serum FGF23 concentrations were measured by ELISA.', 'A 5-year-old French boy with HHS and his family members participated.', 'The patient presented with painful cortical lesions in his leg. Radiographs of the affected bone showed diaphyseal hyperostosis. The lesional tissue comprised trabeculae of immature, woven bone surrounded by fibrous tissue. Biochemistry revealed elevated phosphate, tubular maximum rate for phosphate reabsorption per deciliter of glomerular filtrate, and 1,25-dihydroxyvitamin D levels. The patient was a compound heterozygote for two novel GALNT3 mutations. His parents and brother were heterozygous for one of the mutations and had no biochemical abnormalities. Intact FGF23 level in the patient was low normal, whereas C-terminal FGF23 was elevated, a pattern similar to TC.'] Do novel GALNT3 mutations causing hyperostosis-hyperphosphatemia syndrome result in low intact fibroblast growth factor 23 concentrations?