One of the most remarkable observations is that within each reading frame, selection on single coding regions appears to be more constrained than in double coding ones. As before, we calculate the selection acting on each region as a whole, as shown in Table 2 is in line with the results shown by de Oliveira et al. [2] and more recently by McCauley et al. [13] , but somewhat contrary to general belief [19, 22] . Clearly within the HIV2 genome there is much less data than with Hepatitis B, so it is harder to assign a true significance to these figures. However, our results do appear to suggest less stringent selection on overlapping regions than on single coding ones, thus maybe indicating the overlapping regions to be a relatively young feature in the virus.