In mammalian cells, including those of human beings, DNA-dependent RNA polymerase first synthesises precursor RNA in the nucleoplasm. This precursor is then degraded by a nuclear nuclease to mRNA that is then translocated to the cytoplasm, where it becomes associated to the ribosomal system. This precursor RNA constitutes the fourth class of RNA molecules and is designated as heterogeneous nuclear RNA (hnRNA). Molecules may have molecular weights exceeding 107 daltons, whereas the mRNA molecules are generally smaller than 2 × 106 daltons. Most mammalian mRNA molecules are 400–4,000 nucleotides in length, whereas an hnRNA molecule possesses 5,000–50,000 nucleotides. Thus, the hnRNA molecule appears to be processed to generate the mRNA molecules, which then enter the cytoplasm to serve as templates for protein synthesis.