3' untranslated region 5' untranslated region Alternative Splicing Binding Sites CACO 2 cell line Caco-2 Cells Cystic Fibrosis Transmembrane Conductance Regulator Exons HEK293 Cells Humans Models, Genetic Nerve Tissue Proteins Nuclear Proteins RNA Polymerase II RNA Splice Sites RNA polymerase II RNA-Binding Proteins Ribonucleoproteins Transcription, Genetic Western blotting alternative RNA splicing amplicon article chromatin structure controlled study enzyme inhibition enzyme phosphorylation exon exon skipping expression vector