Corona proteins ratios are also  posttranscriptionally regulated. Most sgmRNAs are structurally polycistronic but functionally monicistronic, with only the 5′-most ORF being translated into a viral protein. The clearest example of coronavirus translational regulation is the expression of the polyprotein pp1ab, which is generated by a programmed-1 ribosomal frame-shifting mechanism. This process leads to minor levels of most of the RNA-modifying enzymes, encoded by ORF1b, in comparison with those of other replicase enzymes, such as proteases, encoded by ORF1a.