Gene therapy is an attractive means to replace a deficient or defective protein. Using a murine retroviral vector, we provide an example of reconstituting a C regulator by neonatal in vivo gene transfer. The fusion gene containing the mouse C receptor 1-related gene/protein y (Crry) and a single chain Ab fragment with specificity for mouse glycophorin A was placed under transcriptional control of a liver-specific promoter. Shortly after birth, Crry KO mice were injected with the retroviral vectors. Protein expression progressively increased over the next 6-8 wk after which an equilibrium was established. Coating levels on RBCs were obtained that inhibited C activation similar to wild-type cells and remained constant for > 1 year. Thus, gene therapy with targeted regulators represents a treatment option to provide a long-term and sustained protein supply for the site-specific blockade of undesirable complement activation.