The Hepatitis B virus PREalpha (HBV PREalpha) is an RNA structure that is shown to play a role in nuclear export of HBV mRNAs.
HBV PREalpha consists of a 30 nt stem-loop, with a 5 nt apical loop. The conserved stem-loop was predicted within the HBV PRE sequence and confirmed by mutagenesis. The Rfam structure is based on a covariation model and is consistent with its solution structure.
The exact role of this structure in nuclear export has not yet been determined 
^Chen, A; Panjaworayan T-Thienprasert, N; Brown, CM (7 July 2014). "Prospects for inhibiting the post-transcriptional regulation of gene expression in hepatitis B virus.". World Journal of Gastroenterology : WJG. 20 (25): 7993–8004. doi:10.3748/wjg.v20.i25.7993. PMID25009369.