There have been many reports of in vivo direct gene transfer methods, but there are problems with these methods. Recently, we developed a novel, nontoxic and efficient in vivo direct gene transfer method mediated by Sendai virus (HVJ) and liposomes. In our system, foreign genes and nuclear proteins were encapsulated into the same liposomes, which were then treated with inactivated HVJ. The properties of our system are that HVJ enables foreign genes to be introduced directly into the cytoplasm by membrane fusion and that nuclear proteins transport the foreign genes rapidly into the nuclei. In the present study, we succeeded in introducing and expressing the functional gene for human insulin, and we observed the expression of this gene in mouse plasma and the reduction of plasma glucose. Our system will provide a new method of gene transfer aimed at postnatal gene therapy in various diseases.