The ADAR enzyme – adenosine deaminase

Targeted RNA editing is performed by enzymes called adenosine deaminases, or "ADAR" for short, which act as "editors" of RNA. Simply put, ADAR can change one "letter" in the RNA sequence to another. Most edits performed by the ADAR enzyme occur in non-coding RNA. 

The enzyme is particularly active in RNA regions resembling those of viruses, altering them to distinguish between the body's cells and viral cells. When there is a malfunction in the ADAR enzyme, it can lead to autoimmune diseases (if the enzyme fails to alter body cells that resemble viruses, causing them to be mistakenly identified as viral cells) or cancer (if the enzyme acts on tumor cells, leading to their mistaken recognition as normal body cells).