Tyrosine-modified PEI: a novel and highly efficient vector for siRNA delivery in mammalian cells.

Delivery of synthetic small interfering RNA (siRNA) into cell remains the major obstacle to its biological activity. Cationic polymers, and in particular the "proton sponge" polyethylenimine (PEI), has shown promise for cancer gene therapy but appears inefficient for siRNA delivery. Here we report that modifying branched PEI with amino acids led to efficient siRNA delivery into mammalian cell lines, even in the presence of serum, and at dose as low as 1 nM.