fibronectin Animals article nonhuman Humans RNA Polymerase II RNA polymerase II alternative RNA splicing exon human in vivo study priority journal Alternative Splicing Models, Genetic RNA Splicing RNA splicing Transcription, Genetic controlled study genetic transcription promoter region transcription regulation Amino Acid Sequence Coupling Exons Fibronectins HeLa cell Hela Cells Mammalia RNA RNA Precursors