Acetylation of a 25-kDa branched PEI (acPEI) has been shown to decrease the surface charge of resulting polyplex particles, improve gene delivery, and reduce vector cytotoxicity. More recently, we showed that even minor succinylation (as low as 2% of bPEI amines) results in dramatically increased transgene expression and gene knock-in efficiency. Polyplexes formed with 2% zPEI resulted in transgene expression at least 250-fold higher than unmodified bPEI and 50-fold higher than commercial bPEI-PEG upon gene delivery in the presence of serum proteins.