Eukaryotic mRNAs are monocistronic. The primary transcript in eukaryotes is much larger than the mature mRNA and is called heterogeneous nuclear RNA (hnRNA). It contains unique sequences and has about 10 times as many sequences as the mature mRNA. hnRNA undergoes processing in the nucleus, by the phenomenon known as post-transcriptional modification, and finally, the mRNA is produced, and therefore, it is called “mRNA precursor” or “pre-mRNA”.