Based on the presence or absence of increased PTH serum concentration, parathyroid cysts are categorized as functional or secreting (hemorrhage or cystic degeneration of a parathyroid adenoma), and non-functional or non-secreting (true) parathyroid cysts (from embryologic remnants, a coalescence of microcysts or the result of abnormal retention of PTH). The gene discussed is PTH; the disease is parathyroid gland adenoma.