The Period/Cryptochrome (PER/CRY) complex is a heterodimer that transcriptionally regulates circadian rhythm. When it is phosphorylated, it is transported to the nucleus and it inhibits a transcription factor that regulates the synthesis of its own subunits (negative feedback).

Research on the circadian molecular clock speaks of an accumulation of PER/CRY complex in the cytoplasm during the day phase (see review The circadian clock: pacemaker and tumour suppressor, Fu and Lee, 2003). Why does this complex not move to nucleus during the day? In other words how is the phosphorylation-dephosphorylation switch controlled?

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    $\begingroup$ Welcome to Biology SE! In order for people to grasp your question, please clarify the question by explaining the acronyms (i.e., get rid of them :) and work a bit on the grammar as it is very hard to read. a citation may help a big deal as well. $\endgroup$ – AliceD Dec 17 '14 at 10:42
  • $\begingroup$ With the edit, the answer is there in the question itself. $\endgroup$ – WYSIWYG Dec 18 '14 at 15:59
  • $\begingroup$ @WYSIWYG Maybe my edit wasn't clear--PER/CRY builds up in the cytoplasm during the day because it is not phosphorylated (and therefore not transported to the nucleus). At night, it is phosphorylated and negatively regulates itself. I know the answer has something to do with blue light and Cryptochrome but I don't actually know the reason $\endgroup$ – Luigi Dec 18 '14 at 16:52
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    $\begingroup$ @Luigi added an edit to mention the phosphorylation $\endgroup$ – WYSIWYG Dec 18 '14 at 17:09

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