Some genes are too large for a single virus to carry. These gene vectors are very selective. They don't deliver to every cell. In some cases, the gene never arrives in the part of the cells where it can function. This method has its advantages, however.
Some viruses inject their DNA only into certain types of cells.
Suppose, for example, we had a harmless virus that only injected DNA into heart cells. We would then have a way of delivering genes to just the heart. Liposomes Another approach essentially consists of encasing genes in fat. Normal cells in the body are always enclosed by a thin layer of fat molecules, rather like a soap bubble. By enclosing genes in little bubbles of fat called liposomes, scientists found that the two fatty layers would melt together, like two soap bubbles merging into one.