Both viral and non-viral methods of gene delivery have proven effective in transfecting patient cells in the clinic and producing detectable levels of therapeutic protein.

A lack of clinical efficacy, however, has led most scientists to conclude that the dose of therapeutic protein has been too low. Consequently, recent developmental efforts have focused on achieving higher effective doses. Unfortunately, physical dose sizes are already close to a maximum, based upon toxicity or manufacturing limitations, and would not appear to offer much opportunity for advancement.