DM1 is caused by pathogenic expansion of an unstable CTG microsatellite in the 3′ UTR of the DM1 protein kinase (DMPK) gene, transcribed to an RNA messenger with an expanded CUG (CUGexp), leading to the formation of ribonuclear foci in the nuclei of affected cells, the most prominent histopathological hallmark of the disease. The gene discussed is DMPK; the disease is myotonic dystrophy type 1.