Krebs von den Lungen 6 (KL-6) is a mucin-like, high molecular weight glycoprotein expressed on the surface membrane of alveolar epithelial cells (AEC-II) and bronchiolar epithelial cells. It exists as a solubilized component in the pulmonary epithelial lining fluid by proteolytic cleavage. KL-6 serum concentration is elevated mainly because two mechanisms: Elevated pulmonary production due to diffuse hyperplasia of alveolar epithelial cells (AEC, called also pneumocytes).