ATXN1 and juvenile Huntington disease: Tissue-specific somatic instability has been identified in a few TREs associated with other neurodegenerative diseases such as Huntington’s disease, where the CAG repeat shows a lower instability in spinal cord and cerebellum, or SCA1 and SCA3 with a lower degree of mosaicism found in the cerebellar cortex for the CAG repetitive tracts compared to other CNS regions (Cancel et al. 1998; Kraus-Perrotta and Lagalwar 2016; Mouro Pinto et al. 2020).