 High-resolution immunoelectron microscopy has shown that hnRNPs localize predominantly to the border regions of chromatin, where it has access to these nascent RNAs.The proteins involved in the hnRNP complexes are collectively known as heterogeneous ribonucleoproteins.They include protein K and polypyrimidine tract-binding protein (PTB), which is regulated by phosphorylation catalyzed by protein kinase A and is responsible for suppressing RNA splicing at a particular exon by blocking access of the spliceosome to the polypyrimidine tract.