H-DNA-forming repeats have been implicated in four REDs: Friedreich's ataxia, GAA-FGF14-related ataxia, X-linked Dystonia Parkinsonism, and cerebellar ataxia, neuropathy and vestibular areflexia syndrome. In this review, we summarize H-DNA's discovery and characterization, evidence for its existence and function in vivo, and the field's current knowledge on its role in physiology and pathology.