The 5' terminal m7G cap present on most eukaryotic mRNAs promotes translation, in vitro, at the initiation level. For most RNAs, the cap structure increases stability, decreases susceptibility to exonuclease degradation, and promotes the formation of mRNA initiation complexes. Certain prokaryotic mRNAs with 5' terminal cap structures are translated as efficiently as eukaryotic mRNAs in a eukaryotic cell-free protein synthesizing system. Splicing of certain eukaryotic substrate RNAs has also been observed to require a cap structure