To further examine the role of the loop identity or size in ribosomal frameshifting, we cloned some of the above loop mutants into a dual-luciferase reporter plasmid and assayed their frameshifting efficiency in mammalian cells ( Figure 5B ). Our data show that the effects of loop nucleotides are comparable in vitro and in vivo. The stable GAAA tetraloop construct again had the lowest frameshifting efficiency ( Figure 5B , 2.9%), which was half that of the UUCG construct ( Figure 5B, 6 .1%).