I am trying to grow some mouse derived tumor cells (benign tumor with just one flox/flox gene recombination) (identified by GFP expression due to Cre recombination - model well defined) in sphere culture (well defined in literature) to identify sphere forming stem cells. But unfortunately it is not the GFP+ but the GFP- cells that are forming spheres in culture that grow upto passage 10. Whereas the GFP+ cells stop forming spheres by passage 2. Given there is no technical error- I was wondering what some possible theoretic explainations to this situation would be.