Allergy has a pivotal role in specific phenotypes of CRS: in allergic fungal rhinosinusitis (AFRS), a severe eosinophilic polypoid CRS potentially requiring surgery and characterized by the presence of thick eosinophilic mucin with non-invasive fungal hyphae within the sinuses, hypersensitivity to fungi (such as Bipolaris, Aspergillus, Alternaria, Curvularia, and Drechslera species) is a key point [18,58]. The gene discussed is MUC5AC; the disease is congenital rubella syndrome.