These polymers are synthesized from naturally occurring monomers (amino acids) which can degrade into nontoxic components. Although poly(amino acid)s such as polylysines are synthesized by polymerization using conventional methods, these materials are usually immunogenic and exhibit poor mechanical properties.21 To overcome such problems, the monomeric amino acid units such as lysine or glutamic acid are modified in their side chains to produce polymers with varying mechanical properties. Copolymers of l-glutamic acid and -ethyl l-glutamate with varying ratios of monomers, for example, have been fabricated to deliver a wide variety of drugs with varying release profiles.22 Because of the stability of the peptide bond in water, biodegradation of these polymers occur by dissolution of the intact polymer chains and subsequent enzymatic hydrolysis in the liver or other tissues. Poly(amino acid)s are also synthesized to contain nonpeptide bonds (referred to as pseudo poly(amino acid)s,20 e.g., poly(serine ester)). There are several examples of the use of poly(amino acid)s and pseudo poly(amino acid)s in drug and nucleic acid delivery. Some of the most widely applied polymers include poly[n-(2hydroxyethyl)-l-glutamine] (PHEG),23 b-poly(2-hydroxyethyl aspartamide) (PHEA),24 poly(glutamic acid),25 poly(aspartic acid),26 and polylysine.27