Like other eukaryotes, the 40S ribonucleoprotien contains one 18S rRNA and 32 ribosomal protiens. These and other rRNA structures come from a single 35S transcript. This transcript is synthesizes by polymerase I, while pre-RNA is transcribed by polymerase III. The pre-RNA is later packaged in a 90S RNP. These processes are mediated by endoribionucleases and exoribionucleases. After assembly, 66S particles are released into the nucleoplasm, mature, and are exported to the cytoplasm. However, Woolford Laboratory researchers have discovered that there are still other steps before ribosomal subunits are able to facilitate protein synthesis. They have also noted non-ribosomal molecules, which are necessary for processes such as rearrangement of rRNA structure as well as RNA cleavage and processing.