Gene or drug carriers with charge-conversional properties were considered to be appealing and practical for in vivo application [1,2].A biodegradable pH sensitive copolymer OEI-poly(aspartic acid-b-lysine) (OEAL) was designed and synthesized as the charge shielding system for gene delivery (Fig.1).The OEAL was negatively charged at pH 7.4 and quickly changed to positive charge at pH 6.8.Owing to the excellent charge-conversional function of OEAL responding to the surrounding pH,a ternary complex formed by OEAL,PEI25k and DNA was utilized as the gene delivery system for in vivo investigation.