I am transducing H9 human ES cells with a lentiviral shRNA vector that contains a CMV promoter controlling shRNA and a tGFP marker. For some reason I see no GFP expression in my ES cells, but successful infection and positive GFP in other cell types (MEFs, HEK293T). I was wondering does anyone have an explanation why GFP might be silenced specifically in hES cells? I am fairly sure that the infection is working but would like to see GFP+ colonies.