DMPK and myotonic dystrophy type 1: Given that some of our previously genetically and clinically characterized DM1 patients with repeat interruptions appeared to have later age at onset than expected for the corresponding expansion size (Pešović et al., 2017), herein we characterized mutational dynamics of interrupted DMPK expansions in somatic cells and examined, for the first time, whether repeat interruptions modified age at onset by influencing the level of somatic instability.