The first group with FGF23‐dependent hypophosphatemia (FDH)—having low TmP/GFR, high cFGF23, normal renal function, and normal or mild PTH elevation—was recruited to assess the effect of elevated cFGF23 upon TmP/GFR in the presence of variable PTH activity. The gene discussed is FGF23; the disease is focal dermal hypoplasia.