Gene therapy has been regarded as a potential treatment for many diseases, including inherited or acquired ones and cancers. Safe, efficient and stable expression of exogenous genes in target cells is the key issue of gene therapy, which depends on the gene delivery system remarkably. Generally, there are two types of gene carriers: viral and non-viral vectors. The transfection efficiency of viral vectors is always higher than that of nonviral ones, but the side effects of viral vectors, such as immune response and random insert of the genes into the host chromosomes, are inevitable during the gene therapy. As the new gene transfer system, non-viral vectors make up for the defects of viral ones and play an irreplaceable role in gene therapy. With the emergence and vigorous development of nanotechnology, vectors based on nanomaterials named nanocarriers have gained more and more attentions for their potential advantages in gene delivery: easily being prepared and multifunctional modified; possessing good biocompatibility and causing less immune response; easily passing through tissues and being absorbed by the target cells with higher gene delivery efficiency; effectively protecting the exogenous genes from being quickly destroyed by the enzymes in tissues or cells. In the review, we mainly focus on the investigations of nanocarriers based on metal, inorganic and non-metal, cationic polymer and liposome nanoparticles and prospect the potential development of the nanocarriers.

Contents
1 Introduction
2 Nanocarriers based on metal nanoparticles
2.1 Magnetic nanoparticles
2.2 Gold nanoparticles
3 Nanocarriers based on inorganic and non-metal nanoparticles
3.1 Carbon nanotubes
3.2 Silica nanoparticles
4 Nanocarriers based on cationic polymers
4.1 Polyethyleneimine nanoparticles
4.2 Chitosan nanoparticles
5 Nanocarriers based on liposomes
6 Conclusion