PHEX and X-linked dominant hypophosphatemic rickets: The normal PHEX protein has 10 conserved cysteine residues.2 The truncated protein loses 5 cysteine residues, which are likely involved in disulfide bond formation and is expected to cause a defective secondary protein structure and enzymatic activity.10 The detection of the aberrant mRNA transcript and the deduced loss-of-function PHEX protein provides evidence for the pathogenic role of this novel c.1080-2A>C splice-site mutation in causing the XLHR.