Compaction extent of polyplexes was successfully regulated by cold treatment in buffer solution using thermoresponsive gene earners composed of linear poly(ethyleneimme) (1-PEI) and alkyl side chains Plasmid DNA (pCMV-Luc) was transfected to COS-1 cells using these carriers with different cold treatments The luciferase expression was greatly enhanced when cells were treated at 4 degrees C in well-defined timing This was a direct observation of how intracellular destabilization in regulated timing is important for nonviral gene transfection