Ribouridine, or uridine (rU) is a purine ribonucleoside, and is one of the four standard ribonucleosides that compose an RNA molecule. The presence of the –OH group at the 2’-position of the ribose results in RNA being less stable to DNA (which lacks –OH groups at this position), because this 2’-hydroxyl group can chemically attack the adjacent phosphodiester bond in the sugar-phosphate backbone of RNA, leading to cleavage of the backbone structure. rU forms a Watson-Crick base pair with rA (riboadensoine/adenosine) in RNA duplexes, and dA (deoxyadenosine) in RNA-DNA duplexes.