MSH3 is a genetic modifier of somatic repeat instability in X-linked dystonia parkinsonism

MSH3 modifies both age of onset and the somatic instability of the X-linked dystonia parkinsonism hexanucleotide repeat. This highlights a critical role of somatic instability in driving disease, highlights MSH3 as a human-validated therapeutic target, and underscores somatic instability as a common mechanism among repeat expansion disorders.