Organization of Pro and Pol proteins. Schematic representations of the mature Pro and Pol proteins and their precursors are drawn for examples from several retroviruses. The sequences representing the mature proteins PR, RT, IN, and in two viruses DU are indicated. Rectangles, precursor proteins, with the solid vertical lines marking major cleavage sites and thick horizontal bars, mature proteins. DTG or DSG, YMDD indicate the conserved active site residues in PR and RT. The location of the RNase H domain of RT is also indicated.