Heterogeneous nuclear ribonucleoproteins (hnRNPs) are complexes of
RNA and
protein present in the
cell nucleus during
gene transcription and subsequent
post-transcriptional modification of the newly synthesized RNA (pre-mRNA). The presence of the proteins bound to a pre-mRNA molecule serves as a signal that the pre-mRNA is not yet fully processed and ready for export to the
cytoplasm. Since most mature RNA is exported from the nucleus relatively quickly, most
RNA-binding protein in the nucleus exist as heterogeneous ribonucleoprotein particles. After splicing has occurred, the proteins remain bound to spliced
introns and target them for degradation.