NOS2 and Cluster headache: Lastly, there were two additional studies that examined the association of cluster headache with five polymorphic micro-satellite markers in the three different NO synthase (NOS) genes nNOS (NOS1), iNOS (NOS2A), and eNOS (NOS3) (Sjöstrand et al., 2002) and SERPINA1 gene variants F, M, S, and Z (Summ et al., 2010), but they did not detect any association.