In healthy individuals, PCT is produced in the thyroid C cells and is quickly converted to calcitonin, so levels of serum PCT are very low (<0.02 ng/ml), but in bacterial infections in many extrathyroidal tissues (kidney, spleen, adipocytes, pancreas, colon, brain, and lungs), PCT is synthesized yet such parenchymal tissue lacks the processing pathway necessary to convert PCT to calcitonin, thereby increasing PCT levels. The gene discussed is CALCA; the disease is bacterial infectious disease.