Role of Fbx15 in ES cells and its use in assaying for iPS cells (Yamanaka paper and others)?
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