DMPK and myotonic dystrophy type 1: In a DM1 mouse knock-in model, in which CTG expansions were inserted into the 3′ UTR of the mouse Dmpk gene, Nutter et al. (2019) demonstrated that the expanded CTG repeat has more severe effects on muscle progenitor stages such as myoblasts and myotubes because of the higher expression of Dmpk at those stages and therefore higher capacity to sequester proteins leading to significant RNA mis-splicing (Nutter et al., 2019).