Objective To optimize the system of transfecting cells with target genes by polyethylenimine (PEI) so as to increase cell expression. Methods On the basis of PEI/DNA at different mass ratios (0 ∶ 1

0.5 ∶ 1

1 ∶ 1

2 ∶ 1

3 ∶ 1

4 ∶ 1

5 ∶ 1

6 ∶ 1

7 ∶ 1

8 ∶ 1

9 ∶ 1

and 10 ∶ 1)

the ability of PEI to combine and protect DNA was investigated by agarose gel electrophoresis retardation assay. Cell fluorescence expression was analyzed after transfecting PEI/DNA complexes at different mass ratios in

to NIH - 3T3 cells in order to obtain the optimal mass ratio of PEI/DNA complex. Comparison of repeat PEI ∶ DNA ratio of 2 ∶ 1 (w/w)

PEI ∶ DNA ratios of 2 ∶ 1 and 4 ∶ 2 (w/w) of 3 experiment groups of transfection. Repeating transfection was used to increase cell expression. Results  Agarose gel electrophoresis retardation assay indicated that the quality ratios of forming stable PEI/DNA complexes was in a range of 1 ∶ 1 to 10 ∶ 1. Cell expression of PEI/DNA was improved with the increase of PEI ∶ DNA mass ratios and the amount reached the highest at PEI ∶ DNA ratio of 2 ∶ 1 (w/w)

but began to decrease at PEI ∶ DNA mass ratio greater than 2. By the way of repeat transfection

cell expression in the test group (PEI ∶ DNA ratios of 2 ∶ 1) was significantly higher than the other test groups (

P

0.05). Conclusion Cell expression reaches the highest at PEI ∶ DNA ratio of 2 ∶ 1 (w/w) and can be increased by repeating transfection.