Packaging and infection by a lentiviral vector. During production, the lentivirus vector plasmid generates genomic RNA that is transported to the cytoplasm, and virion assembly takes place at the cellular membrane with structural proteins encoded by the helper plasmid (left panel). Upon entry into a target cell, the lentivirus genome is reverse-transcribed in a preintegration complex, which is transported into the nucleus of dividing and nondividing cells. Once in the cell nucleus, the viral integrase (part of the preintegration complex) mediates integration of the vector genome into the host cell genome. Integration into the host genome is not necessary for lentivirus vector-mediated transgene expression as integrase-deficient vectors are capable of mediating robust expression.