MBNL1 and myotonic dystrophy type 1: ,44 To determine whether the observed repeat-length-dependent gradient in missplicing coincides with a change in free nuclear MBNL1 protein in this isogenic cell panel, RNA foci (a hallmark of DM1 cells) were detected by a (CAG)6 probe using RNA fluorescence in situ hybridization (FISH), followed by MBNL1 protein staining using immunofluorescence.