Considering recombination in env in the context of the rest of the HIV genome, it is apparent that env displays the most dramatically variable natural breakpoint distribution of all HIV genes [24, 32] , and it constitutes the only gene within which there is an extended region with limited recombination (Figure 4) . Nevertheless, although less marked, breakpoint distribution patterns reminiscent of those found in env, with alternate clusters and troughs are also identifiable in several other regions of the genome such as gag and pol [32] (Figure 4) . Although little information is presently available either on differential mechanistic predispositions to recombination across these regions, or on the functionality of the resulting products, it is tempting to speculate that underlying rules such as we have defined here for env may also be operational in these other cases.