The PAC1 receptor is more specific to PACAP compared to VIP, and has therefore had the most interest in migraine,100 particularly given initial suggestions that VIP was not implicated in the pain part of migraine,101 although there are suggestions of its role in inducing episodic cluster headache in bout when administered intravenously.102. This evidence concerns the gene ADCYAP1R1 and Cluster headache.