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

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