10 control mechanism for RNA export. For one, incompletely spliced or processed RNAs are retained at their sites of transcription and not released into the nucleoplasm for export. In addition, pools of stable polyadenylated RNAs are enriched in the nuclear splicing factor compartments, although their function remains unclear (Huang et al. 1994). But retention is also emerging as a mechanism for regulation of speciWc RNAs. A non-coding RNA tran- scribed from the mouse Cat2 gene encoding a cationic amino acid transporter is retained in the nucleus via a dedi- cated 3 end containing extensive repeat sequences (Pra- santh et al. 2005). These sequences are responsible for accumulation of the RNA in a nuclear compartment identi- Wed as paraspeckles (Fig. 1c). This retention serves a physi- ological function because upon stress the Cat2 RNA is cleaved, released from paraspeckles and rapidly exported into the cytoplasm for translation, thus ensuring a rapid physiological stress response. Although the Cat2 RNA is to-date the best characterized example of such retention, a similar mechanism is likely at work for the migration-stimu- lating factor (MSF) mRNA whose 3 end resembles that of Cat2 and is retained within the nucleus (Kay et al. 2005). Under conditions of MSF secretion the RNA appears to be released from the nucleus and rapidly translated. A further candidate for control via nuclear retention is the neuron- speciWc gomafu RNA which is retained in intranuclear compartments. Interestingly, gomafu does not seem to encode for any protein and might thus be a non-coding reg- ulatory RNA which is retained in the nucleus (Sone et al. 2007). Conclusions The past few years have seen a dramatic change in how we view the cell nucleus. We have come to appreciate the pres- ence of distinct structural elements within the nucleus, the presence of a multitude of intranuclear bodies and the fact that genomes are non-randomly organized within the nuclear space. 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