The first microarray study performed in cluster headache (Sjöstrand et al., 2006) found the upregulation of several S100 genes coding for calcium binding proteins during the active phase of the disease compared to remission, annexin A3 (calcium-binding), ICAM3, BIRC1 (neuronal apoptosis inhibitory protein), CREB5, and two human leukocyte antigen genes (HLA-DQA1 and HLA-DQB1) that were upregulated in patients compared to controls. This evidence concerns the gene HLA-DQA1 and Cluster headache.