A completely new mutational event associated with human diseases - the dynamic mutation - was discovered in the last decade. The molecular mechanism underlying dynamic mutation involves the expansion and intergenerational instability of a tandem-arrayed nucleotide sequence that acquire a pathological size, despite its polymorphic occurrence in normal individuals. To date, at least fourteen neurological disorders are associated with this phenomenon, including Huntington's disease (HD), dentatorubral and palidoluysian atrophy (DRPLA), spinobulbar and muscular atrophy (SBMA), myotonic dystrophy (DM), fragile X syndrome, FRAXE mental retardation and spinocerebellar ataxias (SCA) types 1-3, 6-8, 12 and 17. The spinocerebellar ataxias comprise a heterogeneous group of severe neurodegenerative-late onset disorders characterized by loss of balance and coordination. Most of the spinocerebellar ataxias exhibit an autosomal dominant pattern of inheritance and are promoted by the intergenerational expansion of a trinucleotide repeat (CAG)n inside the coding region of the respective gene. The expanded segment is translated into an abnormal polyglutamine tract in the protein, leading to the formation of nuclear aggregates that have been considered the basis of the pathogenesis in most of SCA types. One striking characteristic of these diseases is that the gene is expressed throughout the brain and also in other tissues but no pathological consequences are observed, despite the specific cellular degeneration. The characterization of the mutational event has led to the development of specific and sensitive molecular tests for direct DNA analysis, which allow confirmation of clinical diagnostic and an adequate therapeutic indication as well as genetic counseling.