View of the polymerase active site of HIV-1 RT prepared using the information from the structure of HIV-1 RT in a complex with double-stranded DNA (Jacobo-Molina et al. 1993; see also Fig. 5). The numbering of the β sheets and α helices is the same as that shown in Fig. 7. The side chains of the active site residues (Asp-110, Asp-184, Asp-185) are shown as are the positions of two tyrosine residues (Tyr-181, Tyr-188) important in susceptibility to nonnucleoside RT inhibitors (see Chapter 12. Also shown are the last two bases of the DNA primer strand (Pri-17 and Pri-18). The base of the incoming triphosphate would be expected to stack on the base of Pri-18, with the phosphates extending over the three aspartic acid residues. (Reprinted, with permission, from Jacobo-Molina et al. 1993.)