i tried to do transfection with 4 different plasmid in 293T cells. But i can't transfect the 4 plasmids together. I need to express protein A & B from 2 plasmid first. And i need to transfect GFP-LacI and last i need to transfect a LacO reporter plasmid (~20K). I've tried several ways to do this but i can't achieve high expression of proteins. and the transfection efficiency of LacO plasmid is very low (<5-10%). I'm using lipofectamine 2000 the transfection. Can anyone have some good advices on the procedures? thanks !