Types of naturally occurring RNA and their functions in biological systems RNA, a versatile macromolecule that is specialized for many functions, can be broadly defined as coding or messenger RNA (mRNA) and non-coding RNA (ncRNA). There are several different types of ncRNA including ribosomal RNA (rRNA) , transfer RNA (tRNA) , small nuclear RNA (snRNA) ,, small nucleolar RNA (snoRNA) , long non-coding RNA (lncRNA) , short hairpin RNA (shRNA), micro-RNA (miRNA) , transfer messenger RNA (tmRNA), small interfering RNA (siRNA), small activating RNA (saRNA), piwi-interacting RNA (piRNA) , circular RNA (circRNA), ribozymes, and exosomal RNA . The cellular localizations of different types of RNA are illustrated in Figure and an overview of their functions is provided in the later section.