Ribonuclease

Ribonuclease (commonly abbreviated RNase) is a type of enzyme that chops RNA into smaller parts. There are many different versions of the enzyme.D'Alessio G. and Riordan J.F. eds. 1997. Ribonucleases: structures and functions. Academic Press.

All organisms studied contain many RNases of many different sorts. This shows that RNA degradation is a very ancient and important process. 
It cleans up cell RNA which is no longer needed.
It is a first-line defence against RNA viruses.
It acts in RNA interference (RNAi), or post transcriptional gene silencing (PTGS). This is a biological process in which RNA molecules inhibit gene expression. In 2006, Andrew Fire and Craig Mello shared the Nobel Prize in Physiology or Medicine for their work on RNA interference.

 References 

Category:Enzymes
Category:RNA
Category:Immunology