Numerous functional studies and gene-targeted mouse models have shown that epileptogenic mutations cause loss of function of Nav1.1 (Guerrini et al., 2014), whereas we and others have shown that hemiplegic migraine Nav1.1 mutations cause gain of function of the channel (Cestèle et al., 2008, 2013a,b; Fan et al., 2016). This evidence concerns the gene SCN1A and familial or sporadic hemiplegic migraine.