Methods of treatment of complement-mediated disorders, in particular disorders associated with over-activity of the complement C3b feedback cycle (for example, age-related macular degeneration (AMD)), using gene therapy is described. According to the methods, levels of complement Factor I are elevated by administration of a recombinant viral vector encoding Factor I such that a therapeutically effective amount of the encoded Factor I is expressed from the vector in the subject. Recombinant viral vectors encoding Factor I, recombinant virus particles encapsidating the vectors, and their use in the methods of treatment, is also described.
