Yes. I assume that this question is about mice? In general, +/+ is wildype, +/- heterozygote and -/- fully deficient. Please note: A targeted knockout mouse does not always have to be made by fully 'deleting' the gene. It can also be made by inserting a piece of extra DNA into the target gene (often targeting exon 1). This renders the gene useless. In this case, the gene sequence of interest is still present in the mice, but inactive. Additionally, there are also conditional knockouts: these mice express the gene of interest, surrounded by introduced sequences that allow it to be taken out when these mice are crossed with a special second strain. (http://www.ncbi.nlm.nih.gov/pubmed/21080282). In special cases, this deletion-event can even be controlled by the administration of food (called inducible knockout).

For your last question: I do not see a way that it would be helpful to mutate in cells from regular FBW7 -/- mice, even in case when the targeted sequence is not fully deleted. However, it may be of interest when your -/- mice are actually inducible -/- mice (i.e. they can still express FBXW7 until induction). In this case, you will have to induce the mutation in the ES cells of these mice (http://www.ncbi.nlm.nih.gov/pubmed/2554496)

Thank Coen for your answer. It helps a lot!

No problem! Good luck on your work!