HLA-DRB1 and idiopathic pulmonary fibrosis: Other genes with polymorphisms associated with the risk for IPF were expressed in subsets of the various cell populations, including CDKN1A and HLA-DRB1 in antigen-presenting cells, and MUC2 in mucin-producing cells (Fig. 3c, Supplemental Figure S5C, E, G, I).