The low levels of exhaled and nasal NO found in patients with CRS have been attributed to various mechanisms: (1) sinus ostium obstruction and increased mucosal absorption, injury to the NO-producing sinus mucosa by increased production of cytotoxic agents in chronic inflammation, or (3) decreased iNOS expression caused by certain cytokines, such as IL-4, IL-6, and TGF-β, found in the sinus mucosa of patients with CRS [32,33,34]. This evidence concerns the gene NOS2 and congenital rubella syndrome.