In Myotonic Dystrophy Type 1, which genetic repeat is implicated?

Myotonic Dystrophy Type 1 is caused by an expansion of a CTG trinucleotide repeat in the DMPK gene (dystrophia myotonica protein kinase). This genetic mutation leads to the characteristic features of the disease, which include myotonia, muscle weakness, and various systemic manifestations.

The CTG repeat expansion in the DMPK gene affects RNA splicing and leads to toxic gain-of-function effects that disrupt normal cellular processes. This disturbance in cellular function is primarily responsible for the symptoms observed in individuals with this condition.

In contrast, the other choices refer to different genetic disorders: the CCTG repeat in ZNF9 is associated with Myotonic Dystrophy Type 2, the CGG repeat in FMR1 is linked to Fragile X syndrome, and the TAC repeat in CEBPA is not associated with a well-known genetic disorder like the others. Hence, the specific association of the CTG repeat with Myotonic Dystrophy Type 1 makes it the correct answer.