The majority of parathyroid adenomas are composed predominately of chief cells (Figure 1), however there is a subset of hyperfunctioning parathyroid adenomas that contain greater than 90% oxyphil cells and are classified as oxyphil adenomas These tumors, in addition to having a cytoplasm filled with abnormal mitochondria, contain the organelles of protein synthesis and secretion and contain histochemically identifiable PTH. The gene discussed is PTH; the disease is parathyroid gland adenoma.