Recent mapping of functional sequence elements in the human genome has led to the realization that transcription is pervasive and that noncoding RNAs compose a significant portion of the transcriptome. Some dominantly inherited neurological disorders are associated with the expansion of microsatellite repeats in noncoding regions that result in the synthesis of pathogenic RNAs. Here, we review RNA gain-of-function mechanisms underlying three of these microsatellite expansion disorders to illustrate how some mutant RNAs cause disease.