MBNL1 and myotonic dystrophy type 1: To investigate RNA–protein interactions, here is describeda live-cell NanoBioluminescence Resonance Energy Transfer (NanoBRET)assay to study the interaction between expanded r­(CUG) repeats [r­(CUG)exp] and muscleblind-like 1 (MBNL1), central to DM1 pathogenesis.This r­(CUG)exp sequesters MBNL1, a regulator of alternativepre-mRNA splicing, in nuclear foci causing splicing dysregulation.In the NanoBRET assay, r­(CUG)exp acts as a scaffold tobring into proximity a BRET pair, MBNL1–NanoLuciferase (NanoLuc)and MBNL1–HaloTag, enabling a quantitative readout of RNA–proteininteractions.