Lipofectamine or Lipofectamine 2000 is a common transfection reagent, produced and sold by Invitrogen, used in molecular and cellular biology.  It is used to increase the transfection efficiency of RNA (including mRNA and siRNA) or plasmid DNA into in vitro cell cultures by lipofection. Lipofectamine reagent contains lipid subunits that can form liposomes in an aqueous environment, which entrap the transfection payload, i.e. DNA plasmids.
Lipofectamine is a cationic liposome formulation, which complexes with negatively charged nucleic acid molecules to allow them to overcome the electrostatic repulsion of the cell membrane. Lipofectamine's cationic lipid molecules are formulated with a neutral co-lipid (helper lipid).  The DNA-containing liposomes (with positive charge on their surfaces) can fuse with the negatively charged plasma membrane of living cells, due to the neutral co-lipid mediating fusion of the liposome with the cell membrane, allowing nucleic acid to cross into the cytoplasm and contents to be available to the cell for replication or expression.
In order for a cell to express this transgene, the nucleic acid must reach the nucleus of the cell to begin transcription. This process involves many risks. The transfected genetic material may never reach the nucleus in the first place, instead being disrupted somewhere along the delivery process. In dividing cells, the material may reach the nucleus by being trapped in the reassembling nuclear envelope following mitosis. But also in non-dividing cells, research has shown that Lipofectamine improves the efficiency of transfection, which suggests that it additionally helps the transfected genetic material penetrate the intact nuclear envelope.
- Invitrogen (2012). "Lipofectamine 2000 Reagent." 
- Dalby, B., Cates, S., Harris, A., Ohki, E.C., Tilkins, M.L., Price, P.J., and Ciccarone, V.C. Advanced transfection with Lipofectamine 2000 reagent: primary neurons, siRNA, and high-throughput applications. Science Direct, Methods 33, 95-103 (2004). 
- Yongliang Chu et al. (2009). "Transfection reagents." Patent Number 7,479,573 B2