Somatic expressions can result in earlier onset for some people

Some genes naturally contain repeat sequences in the DNA. For the most part, these DNA repeats are relatively short and do not cause disease. However, when the DNA repeats expand above a certain size, they can lead to diseases. The expansion of DNA repeats is called repeat expansion mutation

Somatic expansions are increases in the size of repeat #DNA sequences that build up in cells over an individual’s lifetime. Longer DNA repeats can result in patients experiencing symptoms earlier in certain types of #Ataxia. For a better understanding of what somatic expressions are, read the new SCAsource article here: Snapshot: What are somatic expansions? - National Ataxia Foundation