MUC5B and rheumatoid arthritis: A common gain-of-function variant in the promoter of MUC5B, which encodes mucin 5B, a glycoprotein component of airway mucus with a role in airway clearance and defense against bacterial infection, has been identified as a risk factor for different types of fibrotic ILD, including IPF, RA-ILD, and chronic HP [7, 38].