ATXN2 and Machado-Joseph disease: SCAs present mutations in 40 different genes many of which carry an expansion of the polyglutamine tract such as SCA1, SCA2, SCA3/MJD (Machado-Joseph disease), SCA6, SCA7, SCA17 and DRPLA (Dentatorubral-pallidoluysian atrophy; Klockgether et al., 2019).