The small ribosomal subunits have been shown to be sensitive towards mRNA secondary structure in the process of translation initiation and mRNA structures can exclude initiation both in eukaryotes during the scanning process (15) and in prokaryotes for binding between the mRNA and the 3 0 -end of 16S RNA (16) . The fully assembled and translating 70S or 80S ribosomes seem to be more robust. It is, however, broadly accepted that mRNA secondary structures can function as obstacles to translating ribosomes (17, 18) although examples exists of large secondary structures in mRNA that are translated without any ribosomal delay (19) . Nevertheless, there is compelling evidence from in vitro experiments showing that ribosomes may pause upstream to such structures, most pronounced if the structures form pseudoknots (20) (21) (22) . Possibly the lack of rotational freedom in the helix of stem 1, due to the pairing in stem 2, makes pseudoknot structures harder to 'unzip' by the ribosome than simple stem-loop structures (23) . This may explain why pseudoknots can pause ribosomes. Examples from nature show the existence of diverse peptide sequences, often present in regulatory circuits, which will stall ribosomes (24), but to our knowledge, a permanent halt of ribosomes caused by mRNA structures has not been shown previously.