Viral vectors remain the focus of intensive research to optimize their effi ciency, to minimize their immuno genicity and to enhance their tissue specifi city [19, 31, 69, 70] . Strategies to develop less immunogenic vectors have focused on modifying the naturally occurring proteins in the viral coat [71] . Much research has been devoted to searching and characterizing both naturally occurring [71] and engineered capsid variants from mammalian species [72] . Capsid protein modification has also been used to enhance tissue specifi city [70] . Envelope protein pseudotyping involves encapsulating the modifi ed genome from one virus, such as simian immuno defi ci ency virus, with envelope proteins from another virus, such as vesicular stomatitic virus. Th is encapsu lation can enhance the therapeutic potential of viral vectors, by combining the advantages of one viral genome (for example, bigger payload or site-specifi c integration) with the tissue tropism of another virus.