SFTPD and idiopathic pulmonary fibrosis: Serum levels of mucin-like glycoprotein Krebs von den Lungen 6 antigen (KL-6) [4], surfactant protein (SP)-A and SP-D [5,6], matrix metalloproteinase (MMP)1 and MMP7 [7], brain natriuretic peptide [8], and, most recently CC-chemokine ligand 18 [9] are elevated in patients with IPF.