SFTPA1 and pulmonary fibrosis: For example, it has been shown that the substitution of the hydrophilic amino acid arginine by the hydrophobic tryptophan within the SP-A1 gene coding sequence (i.e., the SP-A1 6A4 haplotype) may change the biochemical properties of the protein and alter its in vitro behavior, predisposing to the development of idiopathic pulmonary fibrosis [20].