Tetraloop

Tetraloops are a type of four-base hairpin loop motifs in RNA secondary structure that cap many double helices. There are many variants of the tetraloop. The published ones include ANYA,
CUYG, 
GNRA,
UNAC
and UNCG.
Three types of tetraloops are common in ribosomal RNA: GNRA, UNCG and CUUG, in which the N could be either uracil, adenine, cytosine, or guanine, and the R is either guanine or adenine. These three sequences form stable and conserved tetraloops that play an important role in structural stability and biological function of 16S rRNA.