Ribosomal RNA (rRNA) includes non-coding RNAs that play essential roles in rRNA regulation. Ribosomal RNA (rRNA) takes part in protein synthesis. Occasional RNA molecules act catalytically, as RNA enzymes (ribozymes) or take part in protein export. The most important ribozyme is the major rRNA of the large subunit of the ribosome (28s rRNA in eukaryotes). It is now accepted that 28S rRNA catalyzes the critical step in polypeptide synthesis in addition to playing a major structural role.