Although a hairpin is considered to be a less efficient frameshift-inducing secondary structure than a pseudoknot, some viruses like Human immunodeficiency virus (HIV) (16) , Human T-lymphotropic virus type-2 (HTLV-2) (17) and Cocksfoot mottle virus (CfMV) (18) make use of a simple hairpin to stimulate substantial levels of frameshifting. In addition, frameshifting in the prokaryotic dnaX gene requires, next to an upstream enhancer, the presence of a hairpin as well (19) . A few studies have investigated a correlation between hairpin stability and frameshift efficiency of natural shifty hairpins (19, 20) . Nonetheless, certain studies have shown that a hairpin composed of the same base pairs as a frameshifter pseudoknot is not very efficient in inducing frameshifting in mammalian cells and lysates (21) (22) (23) but is in other systems (24) .