The strength of these signals to function as mRNA destabilizing elements should be equal to a combination of (i) their strengths as À1 RF signals and (ii) their abilities to block ribosome progression, i.e. their thermodynamic stability. The EST2 signal is both highly efficient at promoting À1 RF [$55%, see (15) ], and is predicted to be quite stable ($À27 to À24 kcal/mol depending on the particular folding solution, see http:// prfdb.umd.edu/). It is important to note however that the software used to predict mRNA pseudoknots can neither identify base triples, which make major contributions to frameshifting (40) (41) (42) (43) (44) , let alone calculate their contributions to thermodynamic stability. Regardless, this combination of high frameshifting and thermodynamic stability results in very strong destabilization via NMD ( Figure 2B ), and perhaps NGD as well ( Figure 2C ). As discussed previously (19) , the exponential decay profile suggests that NMD can occur beyond the 'pioneer round' of translation.