The abbreviated form for heterogeneous nuclear ribonucleic acid: an extrachromosomal RNA molecule in the nucleus that serve as primary transcript from DNA.
The hnRNA is the collective term for the unprocessed mRNA molecules in the nucleus. It is largely comprised of the pre-mRNA molecules that require extensive processing to become mature mRNA molecules. Although the hnRNA serve as precursor of mRNA, not all hnRNA become cytoplasmic mRNA.
See also: pre-RNA