I am transfecting several B cell lines either with a lentivirus containing a siRNA and a GFP gene or with a virus containing only the GFP. The effect I have observed is that although my transfection rate for both of these viruses is very low (~2% as measured by GFP expression), I get severely reduced cell growth for the cell cultures containing both viruses. So (for example) for the Raji Burkitt's lymphoma line that I have transfected with the GFP only virus I get 3% of the cells expressing GFP but I get a 45% reduction in cell growth over 4 days, for the Raji cells transfected with the GFP and siRNA virus I get only 1.3% of the cells expressing GFP but I get a 68% reduction in cell growth. I find it very surprising that such a small percent of transfected cells would have such a generalised effect on the rest of the cells in the culture. Has anyone else seen this sort of effect?