Upon Northern blotting, Saccharomyces cerevisiae that was treated with diazaborine showed aberrant mRNAs that were extended at the 3'-end and terminated at secondary processing sites. These bands were also detected in untreated Deltaupf1, Deltaxrn1, and rat7-1 mutants. This finding demonstrates that the aberrant mRNAs also occur in untreated strains in small quantities and can reach the cytoplasm, where they are normally degraded by Xrn1p. Diazaborine treatment stabilizes these mRNAs. The detection of the aberrant bands in the untreated rat7-1 strain indicates that Rat7 is involved in quality control of RNA. The aberrant mRNAs were not detected after diazaborine treatment of a DRG1-1 mutant. Drg1p, a member of the family of AAA (ATPases associated with a variety of cellular activities) proteins, which are thought to represent specific chaperones, may be involved in the process of unfolding the mRNA-ribonucleoprotein complex or in the recognition of aberrant mRNA molecules in the cytoplasm.