MUC5B and idiopathic pulmonary fibrosis: In 2011, two laboratories simultaneously reported that the MUC5B promoter variant rs35705950 is the strongest risk factor for the development of f-IPF and s-IPF 128 and appears to be similarly prevalent in patients with f-IPF and s-IPF 117 (approximately 50-60% of individuals with FIP or IPF).