MBNL1 and myotonic dystrophy type 1: Similar, but relatively small amounts of endogenous MBNL1, localize both in nuclear CUG foci (∼8%) in DM1 myoblasts, where aberrant splicing is observed, and in cytoplasmic CUG-foci (∼7%) in α-MHC-LacZ-(CTG)400 cardiomyocytes, where splice defects are not observed [Figure 4].