In many cases, such as in ATP1A3‐ and TUBB4‐related disorders,32, 33 it remains controversial whether phenotypic heterogeneity is due to variable expressivity (ie, phenotypic spectrum of the same disease), pleiotropy (ie, discrete phenotypes related to the involvement of different organs or different neuronal subpopulations in exclusively neurological disorders), or both.34 This evidence concerns the gene ATP1A3 and nervous system disorder.