NOS1 and Headache: The frequency of the major allele C of rs3782218 of the NOS1 gene was statistically significantly lower in group 1 (TTH and AH) compared to group 2 (AH without headache): 73.3% versus 91.7%, respectively (p-value = 0.008) and the frequency of the minor allele T of rs3782218 was statistically significantly higher (3.2 times) in group 1 (TTH and AH) compared to group 2 (AH without headache): 26.7% versus 8.3%, respectively (p-value = 0.008); Table 2.