problems digesting genomic DNA from cell lines - (Aug/06/2006 )

I have been isolating genomic DNA (Gentra kit) from cell lines established from primary mouse bone marrow cells. The cells have been transduced with retrovirus. I can get good DNA yeilds however it is completely resistant to multiple restriction enzymes. I do not have this problem with DNA isolated from fresh tissue. Any ideas/suggestions are appreciated!

Are you sure you have genomic DNA not over-abundance of RNA? How are you testing whether gDNA was obtaineD? running a gel?