Figure 2.

Generation and impacts of endogenous retroviruses. When an exogenous ALV (exALV) infects a sexual cell, it can become endogenous (enALV) and stay integrated in the cell genome (germline transmission) (1). Occasionally RNA can be transcribed (2) and translated in the cytoplasm (3). enALV proteins can be presented on the cell surface as antigens (4), interfere with exALV by blocking the receptors (5), or, along with enALV RNA, assemble into new viruses to be shed from the cell (6). When the cell is infected by an exALV, the resulting DNA may recombine with enALV cDNA (7) or with proviral enALV in the bird genome and give rise to new proviruses (8). If the new provirus suffers deletions or insertions, it will become defective (9). When enALV integrates in the proximity of a cellular gene, the potent promoters in the 3′ LTR may control its expression (10). Cellular processes are shown in italics, the nucleus in light green and the cytoplasm in cream (adapted from [38]).