Overview of repeats
Fuchs endothelial corneal dystrophy type 3 (FECD3) is an eye disease which is caused by CTG repeat expansions in the TCF4 gene (Zarouchlioti et al., 2018). The conservative approach has defined 50 or more CTG repeats as the cut-off for disease-causing (Zarouchlioti et al., 2018). In one research conducted by Wieben et al. (2012), it was reported that 79% of patients had more than 50 repeats (reaching to over 1000 repeats in 4 cases) and 20% of the cases had allele below 40 repeats. In the control group of 63 individuals, 95% of the subjects had below 40 repeats and 5% over 50 repeats (Wieben et al., 2012). However, it was recently recommended that the length of at least 32 repeats should be considered as risk associated (Zarouchlioti et al., 2018).