I have tried to use Lipofectamine 3000 reagent to do transfection, but I encountered some problem that when add plasmid into Lipofectamine3000 Reagent I got cotton-like sediment leading to low transfection efficiency. I thought this segiment is DNA but why? I performed the mix step at room temperature and the OptiMEM was prewarmed to 37℃. How can I solve this problem?