DM1 is caused by the abnormal expansion of a trinucleotide CTG repeat in 3′UTR of the myotonic dystrophy protein kinase (DMPK) gene [15]: in contrast to the short repeats carried by unaffected individuals, pathogenic expansions are longer than 50 CTG repeats, reaching >1000 CTG in the most severe forms of the disease: larger repeats are associated with more severe symptoms and earlier onset [16]. The gene discussed is DMPK; the disease is myotonic dystrophy type 1.