Abstract: The present invention is directed to a design of and a method to synthesize polycations for gene (DNA and RNA) delivery. According to this design, the polycations (also said cationic polymers) are formed by polymerization of endogenous monomers bearing sufficient amino groups through degradable bonds with linker molecules or with themselves. The amino group-bearing monomers are those naturally existing in or nontoxic to human body. The linker molecules are those which are not only degradable to nontoxic fragments but also able to release the amino group-bearing monomers in their native state upon degradation. Some examples for the endogenous amino group-bearing monomers are spermine, spermidine, serine or N,N-dimethyl serine, and histidine. Examples for the degradable chemical bonds formed between the amino group-bearing monomers are carbamate, imine, amide, carbonate, and ester.