1,25-dihydroxyvitamin D [1,25(OH)2D], the bioactive metabolite converted from circulating 25-hydroxyvitamin D (25OHD), is thought to prevent chronic PTH hypersecretion and the eventual development of hyperparathyroidism by stimulating CASR gene transcription, inhibiting PTH gene transcription, and blocking parathyroid cell proliferation (8). The gene discussed is PTH; the disease is hyperparathyroidism.