EXCERPT

Families living with extremely rare neurodegenerative diseases finally received an answer to the cause of their illnesses, thanks to a researcher's hunch and decades of improvements in DNA sequencing technology.

Four different rare diseases are all caused by the same short segment of DNA repeated too many times, a mutation researchers call noncoding expanded tandem repeats. Researchers suspect variations of this type of mutation may cause other diseases that have thus far evaded diagnosis by genetic testing.