A new study maps how RNA-binding protein RBM20 generates isoform diversity through independent transcription start sites, according to the description provided. The findings connect transcriptional architecture to splicing outcomes, placing isoform control at the center of RBM20’s regulatory role. Because RBM20 is implicated in development and disease-relevant splicing programs, the work offers a mechanistic handle for understanding how altered isoform balance could contribute to pathology. The study also provides a framework for probing other RNA-binding protein systems governed by start-site complexity.