The life cycle of HIV-1 starts with viral entry, in which process the virus binds to the CD4 receptor or a coreceptor (CCR5 or CXCR4) with gp120 protein, thereby anchoring itself onto host cell surface, allowing fusion between the cellular and viral membranes. After entry into the cell, the viral nucleoprotein together with the contents, i.e. the genomic ssRNA, is released into cytoplasm. Then, utilizing the cellular nucleotides as the building blocks, double-stranded DNA is generated from the virus genome ssRNA directed by the HIV-1 reverse transcriptase (RT) in a nucleoprotein complex termed the RTC. Together with other viral proteins, the newly synthesized DNA constitutes an integration-competent nucleoprotein complex, migrating into host cell nucleus and mediating integration of viral DNA into host chromatin. Integrated viral DNA, named as provirus, becomes part of host genome and serves as a transcription template for the synthesis of viral mRNA and genomic RNA. Following the synthesis of viral genomes and proteins (Table 3), the viral components are assembled together to produce new virions, the virus particles then bud out of host cell and undergo a maturation step to generate infectious HIV-1.
Considering that HIV-1 is a human pathogen which destructs CD4+ helper T lymphocytes and results in the subsequent loss of immune competence, in order to guarantee the experiments safety, efficient HIV-1-derived lentiviral vectors have been developed with improved biosafety features and regarded as a safe gene delivery tool to replace wild-type HIV-1 (Figure 3) .