What is fos or c-fos expression?

I have been doing some reading on the mechanism of sleep, and right and left in papers I see the mention of fos or c-fos expression. fos expression in a neuron seems to mean that it was activated, but what exactly is fos expression? Precisely what does it does it mean when fos expression is visible? References for detailed explanations are appreciated.

What is fos or c-fos expression?
That's an interesting question. I'm mostly familiar with Fos activity in proliferating cells, particularly fibroblasts, but I'll should be able to shed some light on the their role in neurons. The Fos family contains several genes, c-fos, Fra-1, Fra-2, and FosB. These are all examples of immediate early genes (IEGs), which are upregulated by MAP kinase and ERK singaling and function primarily as transcription factors. In most cell types, upregulation of IEGs occurs in repsonse to mitogenic activation. Upon upregulation, the Fos proteins (being transcription factors) migrate into the nuclues and promote the expression of other genes, 'late genes'. In fibroblasts, these 'late genes' promote cell proliferation and growth, but since few neurons actaully grow and proliferate, the Fos genes must be inducing a different set of late genes in neurons. I would have to do a considerable amount of reading to find out what specific late genes are being promoted and how they tie into the sleep issue, but I think that you should have enough info now to piece together the puzzle.
Reply:C-Fos is a gene.