Retroviruses are the only animal viruses that integrate into the host cell's genome during the normal growth cycle. They utilize an integrase (IN, encoded by the pol gene) that acts in a site-specific manner to join the ends of the viral cDNA to (quasi-random) target sequences in host cell DNA. The linear ds cDNA made in the cytoplasm is transported to the nucleus where it is also found as circles and as integrated DNA. Two forms of circular DNA are generally found: one having a single LTR and one having two LTRs, however, it is now generally accepted that the proximal precursor to the integrated provirus is the linear molecule with two LTRs.