Vector		Cell Targets		Cloning Capacity		Advantages		Disadvantages
Adenovirus		Lung, respiratory tract		7.5 kb		Efficient transfection		Strong immune response
Adeno-associated virus		Fibroblasts, T cells, others		4.5 kb		Transfect many cell types		Small insert size
Retroviruses		Proliferating cells		8 kb		Prolonged expression		Low transfection efficiency
Lentiviruses		Stem cells, proliferating cells		8 kb		Efficient transfection		Related to HIV

Most problems in gene-therapy trials have been traced to the vectors used. For instance, many of the viral vectors eventually cause an immune response or have a cloning capacity too low to deliver the required gene.

New viral vectors and targeting strategies are being developed