NOS2 and Cluster headache: Carriers of the AA haplotype rs2779249 of the NOS2 gene showed a higher risk of developing other primary headaches: migraine (OR = 2.65 [95% CI: 1.34–5.22], p-value = 0.0027) [10,54,55] in the Brazilian population, and cluster headache (χ2 = 5.21; p-value = 0.022) [56] in the Swedish population.