The CCA-adding enzyme acts as a molecular vise (blue, green, purple structure), modifying RNAs with a CCA sequence using a screw-like motion. After the full sequence has been added, a normal RNA cannot turn further and pops out of the enzyme (labeled 2a). But mutations can render the RNA more flexible allowing it to refold on the enzyme (labeled 2b). It is only after a second round of CCA addition that the mutant RNA is released from the enzyme. The CCACCA sequence targets the RNA for immediate degradation in the cell.