Tetraloop

A tetraloop is a four-nucleotide hairpin loop in RNA secondary structure that caps many helices. They can generally be divided into three sequence motifs: the 'UNCG', the 'GNRA' and the 'CUYG' tetraloops. In these sequences, R stands for a purine base (R= A or G), Y stands for a pyrimidine base (Y = U or C) and N indicates the position that can be any base. (N = A, U, G, C).

 References:
i) Correll CC, Swinger K. 2005. Common and distinctive features of GNRA
        tetraloops based on a GUAA tetraloop structure at 1.4 resolution. RNA 9:
        355-363
ii) Jucker FM, Pardi A. 1995. Solution structure of the CUUG hairpin loop: a novel
        RNA tetraloop motif. Biochemistry 34: 14416-14427
iii) Cheong C, Varani G, Tinoco Jr I. 1990. Solution structure of an unusually stable
        RNA hairpin, 5'GGAC(UUCG)GUCC. Nature 346: 680-682