Introduction 2007 2001 2003 2006 2005 2006 2006 2003 1 2007 2007 1 Fig. 1 a green red blue b c b red green c white Bar a b c  A key question in understanding nuclear architecture is how protein and RNAs move within the nucleus. This is a key issue if we are to gain a full appreciation of what the nuclear environment is like. More importantly, the mode of nuclear motion of molecules has physiological implications as it affects how proteins find their target sequences in the genome, how RNAs are exported from the nucleus upon transcription and how proteins are sequestered within the nucleus for regulatory purposes. Some initial insights into these issues are summarized in broad terms here. The motion of RNA and proteins in the cell nucleus 2005 1993 1997 2001 2005 2001 1997 2005 2001 1997 2 −1 2 −1 2006 2004 2006 2004 Trafficking as a means of targeting 2001 2001 2006 1999 2000 2004 2006 2002 2006 2004 2006 Protein dynamics as the key for formation of nuclear compartments 2006 2006 2 Fig. 2 RPA43 Fibrillarin B23  2004 2000 2000 2006 2007 The importance of protein and RNA dynamics in regulatory events The observation of dynamic properties of proteins in the cell nucleus of living cells has suggested that dynamic trafficking is an intrinsic property of proteins and RNAs. The dynamic behavior of proteins and RNAs clearly contributes to their proper function. However, is dynamic trafficking important for physiological regulation? Several observations demonstrate that dynamic trafficking of both proteins and RNA indeed can have regulatory function by several means. 3 Fig. 3 a b blue c  2005 2007 2005 2007 1 2005 2007 2007 2005 2007 2005 2007 2007 2007 1 1997 2006 2004 2007 2006 1997 2007 1994 2005 1 2005 gomafu gomafu 2007 Conclusions The past few years have seen a dramatic change in how we view the cell nucleus. We have come to appreciate the presence 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. One of the most consequential findings has been the realization that just about every aspect of nuclear organization is highly dynamic. Both proteins and RNAs move rapidly within the nucleus, and they only transiently interact with chromatin and nuclear bodies. It is now clear that the dynamic nature of nuclear components is a fundamental property and has implications for how molecules are targeted to their final destinations and for how intranuclear compartments form. Most importantly, it is becoming clear that the dynamic properties of nuclear proteins are critical for various mechanisms of physiological regulation, particularly via sequestration and retention of proteins and RNA. It is likely that these recently discovered examples of regulation by modulation of dynamic interactions within the cell nucleus are only the proverbial tip of the iceberg, and it stands to reason that we should consider the contribution of dynamic interactions and trafficking in any nuclear event we investigate in the future.