Transfection of the pEGFP-N1 vector and the pEGFP-N1-DTA vector into 293T cells. A) Transfection of pEGFP-N1 into 293T cells: high EGFP expression at 48 h post-transfection. B) Transfection of pEGFP-N1-DTA into 293T cells: low levels of EGFP expression. A1 and B1, pictures taken under white light; A2 and B2, pictures taken under UV light; A3 and B3, merged pictures. Magnification, 400×.