Retroviral life cycle. Entry of a retrovirus into a cell is initiated by interaction of its envelope proteins with cellular receptors followed by internalization through membrane fusion or phagocytosis. The vector core is released and the RNA serves as a template for reverse transcription and formation of the preintegration complex (PIC). The PIC gains access to chromatin during mitosis or, in the case of lentiviruses, by ingress through the nuclear membrane. Following integration into host cell DNA, the retroviral genome is expressed in RNA molecules which are transported to the cytoplasm to serve as a template for synthesis of new viral proteins and, in the case of unspliced RNA species, as a substrate for formation of new viral particles.