FIG. 1.

Early steps of the retroviral life cycle. After entry, the RNA genome of the retrovirus or retroviral vector undergoes reverse transcription in the cytoplasm of infected cells. Viral DNA, integrase (IN), and certain viral and cellular proteins constitute the preintegration complex, which is imported into the nucleus (Flint et al., 2004). IN then catalyzes the integration of viral DNA into the host cell DNA. As indicated by the asterisk (*), this process applies only to some retroviruses, such as human immunodeficiency virus (HIV)-1 and avian sarcoma virus (ASV; Flint et al., 2004). Murine leukemia virus (MLV) and MLV-based vectors cannot pass readily through the nuclear pore and access the host cell DNA unless the nuclear envelope is dissolved during mitosis (Roe et al., 1993). IN(n), integrase multimer.