PEI stock solution (1 mg/mL)

Reagent	Add to 100 mL	Note
Polyethylenimine powder	100 mg	1. Filter through 0.22 uM filter membrane. 2. Recommend tested for efficiency by transfecting cells with GFP at a 1:1–1:6 DNA:PEI ratio.
HCl	adjust to pH 7.0
MilliQ water	100 mL

Aliquot 0.5–1 mL to each 1.5 mL tube, and store at −20°C. Thawed solutions can be stored at 4°C for up to 2 months. Avoid frequent freeze-thaw cycles.