Several examples have come to light in which mutations in non-protein-coding regions give rise to a deleterious gain-of-function by non-coding RNA. Expression of the toxic RNA is associated with formation of nuclear inclusions and late-onset degenerative changes in brain, heart or skeletal muscle. In the best studied example, myotonic dystrophy, it appears that the main pathogenic effect of the toxic RNA is to sequester binding proteins and compromise the regulation of alternative splicing. This review describes some of the recent advances in understanding the pathophysiology of RNA-dominant diseases.