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I am trying to understand the assay for iPS cells in the Takahashi & Yamanaka 2006 paper.

They inserted a beta-geo cassette, which contains the neomycin resistance gene, into the Fbx15 gene. The idea is that the beta-geo cassette will be expressed when Fbx15 will be expressed, thus conferring G418 resistance to only those cells expressing Fbx15.

They say that "Although specifically expressed in mouse ES cells and early embryos, Fbx15 is dispensable for the maintenance of pluripotency and mouse development." Does this mean that Fbx15 will be expressed in induced pluripotent stem cells, and won't be expressed upon differentiation? (What is special about Fbx15 that will distinguish iPS cells from somatic cells, including those that might derive from iPS cells, as when pluripotency is not maintained?)

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Another paper from Yamanaka's group explains Fbx15.

It says:

Inactivation of Oct3/4 in ES cells led to rapid extinction of Fbx15 expression.

Fbx15-null ES cells were normal in morphology, proliferation, and differentiation. These data demonstrate that Fbx15 is a novel target of Oct3/4 but is dispensable for ES cell self-renewal, development, and fertility.

So Fbx15 wont be expressed in Oct4⁻ cells.

I haven't seen any paper describing the function of this gene but in the case of cassette insertions the locus would prove useful in expressing the inserts specifically in Oct3/4⁺ cells

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So, Fbx15 is a nonessential marker of iPS cells? As in, it's expressed only in iPS cells, but disrupting the gene with a cassette isn't expected to have any effect on the iPS cell? –  dd3 Apr 24 '13 at 5:37
    
Seemingly. [as per the paper] –  WYSIWYG Apr 24 '13 at 5:46
    
Ok thanks. I don't think it's going to get much clearer than this. –  dd3 Apr 24 '13 at 5:47
    
there are only 9 reports on this gene in pubmed. Only one tries to attribute some function. It is in cancer cells, though: FBXO15 regulates P-glycoprotein/ABCB1 expression through the ubiquitin-proteasome pathway in cancer cells –  WYSIWYG Apr 24 '13 at 5:54
    
@dd3 I don't know if Fbx15 is so much a marker of iPS cells as it is just a target of the Oct4 transcription factor, meaning its expression will be active in Oct4+ cells, providing a convenient way of driving exogenous (and then endogenous) expression of the Yamanaka factors long-term. –  MattDMo Apr 24 '13 at 13:37
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