Cells transfected with PEI are primarily neurons. Cells were cultured as described in "Methods." On the fifth day in vitro, cultures were transfected with the plasmid encoding EGFP. Eight days later, the cultures were immunostained with an antibody to the neuronal marker MAP2. (A) shows a cluster of three cells, with the one on the right expressing EGFP (green). (B) indicates that the cells in (A) are MAP2+ (red). Scale bar = 20 μm.