The efficiency of À1 PRF determines the relative ratio of structural Gag to enzymatic Gag-pol available for viral particle self-assembly, and changing À1 PRF strongly inhibits virus maintenance (18, 40) . Previous studies have identified numerous L3 mutants that promoted altered rates of L-A promoted programmed À1 ribosomal frameshifting (À1 PRF), but did not affect Ty1 mediated programmed +1 ribosomal frameshifting (+1 PRF) (13, 17, 19, 23) . However, while all previous L3 mutants analyzed to date enhanced À1 PRF efficiency, all of the viable L3 basic thumb mutants promoted decreased À1 PRF, ranging between $55% and $75% of wild-type rates ( Figure 2C , summarized in Table 1 ). The significance of these changes in À1 PRF is confirmed by loss of the Killer virus, maintenance of which is known to be sensitive to even small decreases in À1 PRF rates (18, 41) . Consistent with prior studies none of the basic thumb mutants affected +1 PRF (summarized in Table 1 ).