Expansions of simple tandem repeats are responsible for almost 50 human diseases, the
majority of which are severe, degenerative, and not currently treatable or preventable. In this
review, we first describe the molecular mechanisms of repeat-induced toxicity, which is the
connecting link between repeat expansions and pathology. We then survey alternative DNA
structures that are formed by expandable repeats and review the evidence that formation of
these structures is at the core of repeat instability. Next, we describe the consequences of …