TCF4 and Fuchs endothelial corneal dystrophy: Fuchs endothelial corneal dystrophy (FECD) is the most common repeat expansion-mediated disease reported to date in humans, predominantly attributed to expansion of a non-coding CTG element (CTG18.1) in the ubiquitously expressed gene, TCF4. At the onset of this study, the contribution of somatic instability to this common, tissue-specific and sight threatening disease was unknown.