MUC1 and idiopathic pulmonary fibrosis: However, accumulating evidence shows that blood levels of certain lung-specific macro-molecules, such as Krebs von den Lungen-6 (KL-6, also known as mucin-1, a glycoprotein expressed on the cell membrane of type 2 alveolar cells) and surfactant protein A (SPA, also secreted by type 2 alveolar cells), are also correlated with the severity of fibrosis and may serve as biomarkers for determining the clinical aspects of IPF [36–40].