Linear polyethylenimine (PEI, molecular weight 40,000) is a positively charged polymer widely used as a transfection reagent in in vitro cell biology and molecular biology studies. PEI condenses negatively charged DNA molecules into positively charged complexes, thereby facilitating their binding to negatively charged proteoglycans and other surface residues on the cell membrane, leading to subsequent cellular uptake through endocytosis. PEI-mediated transfection is compatible with serum-containing media, and depending on specific experimental conditions, transfection efficiency is generally over 60% to 80%. Commonly used transfection cell lines include HEK-293, HEK293T, CHO-K1, HepG2, and HeLa cells. This reagent supports applications ranging from small-scale (96-well plates) to large-scale expression systems (up to 100 liters bioreactors) and is widely used in transient expression experiments for recombinant protein production and functional gene studies.