At first glance, the title of this minireview might appear to be an oxymoron. How can "stable" RNAs be degraded? We have become accustomed to thinking about the stable RNAs, primarily rRNA and tRNA, as being stable in an absolute sense; however, this terminology really is correct only in comparison to the rapid turnover of mRNAs. In fact, although rRNA and tRNA are quite stable during exponential growth (1), it has been known since the earliest studies of RNA metabolism that under certain

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... logical conditions these RNA molecules can be extensively degraded (2). Yet, in contrast to the many studies of mRNA decay (3-5), there has been relatively little attention in recent years to the study of the processes that result in degradation of stable RNAs despite the fact that these molecules account for ϳ98% of all cellular RNA (6).