MBNL1 and myotonic dystrophy type 1: MBNL1 was observed in the nucleus and cytoplasm of CTRL hiPSC-CMs, whereas MBNL1 punctate staining (indicated by red arrows) was colocalized with nuclear CUG foci in DM1-1290 and DM1-1640 hiPSC-CMs (Figure 1A).