While the pathophysiologic mechanism in atopic asthma involves up-regulation of Th2 cytokines, mast cell– and eosinophil-driven inflammation, plus increased activity of inducible nitric oxide synthase (iNOS) and arginase in airway epithelium resulting in obstructive changes and AHR, the exact mechanisms of AHR, obstructive and restrictive lung disease in SCD is unclear. The gene discussed is NOS2; the disease is atopic asthma.