Abstract
Eukaryotic mRNA is processed by enzymes and packaged with proteins within nuclei to generate functional messenger ribonucleoprotein (mRNP) particles. Processing and packaging factors can interact with mRNA cotranscriptionally to form an early mRNP. Erroneous mRNP formation leads to nuclear retention and degradation of the mRNA. It therefore appears that one function of cotranscriptional mRNP assembly is to discard aberrant mRNPs early in their biogenesis. Cotranscriptional mRNP assembly may also enable the transcription machinery to respond to improper mRNP formation.