The complete process of gene therapy involves three important steps: targeting, delivery, and gene expression. Since each step can be related to the pharmacological concept of affinity, bioavailability, and intrinsic capacity, this commentary examines, from this perspective, the efficiency of anionic and cationic liposomes as vectors for the in vivo gene transfer of the human alpha1-antitrypsin gene. Small liposomes represent the first generation of liposomes destined for the liver parenchymal cell. Although the final efficiency of gene transfer is low, we found that small liposomes are a kind of high-affinity hepatocyte-destined vector because the dose range for mediating the response is three orders of magnitude lower than that used by other procedures. Encapsulated DNA is more efficient than the cationic liposome-DNA complex for in vivo gene transfer. This could be due to gene bioavailability, since encapsulated DNA is protected from enzymatic digestion, whereas DNA externally associated with the liposome can be digested before the complex reaches the target cell. However, when the gene transfer efficiencies of anionic and cationic small liposomes were compared, we observed a similar rate of efficiency and potency, since equivalent plasma levels of human protein were observed after the same i.v. dose of recombinant plasmid encapsulated in anionic or cationic liposomes. On the other hand, the elements selected for constructing the expression cassette greatly influence gene expression and the stability of the gene product, and, therefore, the final efficacy is also limited by the intrinsic capacity of a specific expression cassette to express the gene product.