This has led us to take the following approach to the location of tumors causing TIO: First, we perform functional imaging (ie, PET/CT and pentetreotide/CT), followed by confirmation with high-resolution anatomic imaging (ie, CT and/or MRI); if discrimination is needed between multiple suspect lesions indentified on functional imaging or if greater certainty is needed that the identified lesion is in fact the FGF-23-secreting tumor, selective venous sampling and/or aspiration/biopsy with determination of the FGF-23 concentration in the washings is performed. The gene discussed is FGF23; the disease is neoplasm.