MUC5B and idiopathic pulmonary fibrosis: The common variant rs35705950 in the promoter region of MUC5B is carried by 9% of the European population and is the strongest risk factor for developing IPF accounting for 30–35% of the risk and also predicting asymptomatic mild fibrosis.68, 77, 78, 79 There are multiple potential explanations for how this polymorphism could lead to fibrosis (for a discussion, Evans et al. 68), one of which is the ER stress pathway.