To correct a genetic defect, it would not be enough, say, to inject the missing enzyme, since the body's immune defenses would rapidly destroy it. A“Trojan horse”is needed to evade immune surveillance. The evolution of such an approach is described, as well as how it has been used to“cure”Tay-Sachs disease in culture, utilizing immunoglobulin-disguised liposomes to bring hexosaminidase A to deficient cells.