This conversion can be induced by the hormone PTH, while CYP27B1 expression is inhibited by FGF23 and by 1,25(OH)2D itself.14 1,25(OH)2D production may be indirectly increased in the setting of hypocalcemia and hypophosphatemia or decreased in the setting of hypercalcemia and hyperphosphatemia, likely via corresponding changes in PTH and FGF23. The gene discussed is PTH; the disease is hyperphosphatemia.