Structural comparison of a retrovirus with retrotransposons found in eukaryotic genomes. (a) A retrovirus, Moloney murine leukemia virus (MoMLV), of mice. (b) A retrotransposon, Ty1, in yeast. (c) A retrotransposon, copia, in Drosophila. (d) A long interspersed element (LINE) in humans. Abbreviations: LTR, long terminal repeat; ORF, open reading frame.