Repeat expansions of the CTG trinucleotide (50-10,000) in the non-coding 3′-UTR of the myotonic dystrophy protein kinase gene (DMPK) and of CCTG repeats (75-1,100) in intron 1 of the Zinc Finger Protein 9 gene (ZNF9) lead to myotonic dystrophy (DM1) and myotonic dystrophy type 2 (DM2) respectively. The gene discussed is DMPK; the disease is myotonic dystrophy.