(A) SIV V1-V2 variants present in three replicate serial dilutions of plasma from donor monkey 21743 (week 2 p.i.). Lanes 1 to 4, dilution series A (undiluted to 10−3); lanes 5 to 7, dilution series B (undiluted to 10−2); lanes 8 to 11, dilution series C (undiluted to 10−3). The highest dilutions yielding a RT-PCR product (10−2 and/or 10−3) comprised homogeneous variant populations (i.e., single variants) as depicted by the presence of a single homoduplex band (lanes 4, 7, 10, and 11). These V1-V2 sequences (∗) were considered to be the most common envelope variants in the plasma of 21743 at 2 weeks p.i. The variants depicted in lanes 7 and 11 are the same as determined by the formation of a homoduplex when the two variants were mixed together (B) Therefore, there were three major variants present at similar frequencies in the plasma of 21743 at week 2 p.i. (B) HMA gel showing mixtures of the endpoint variants from three dilutions (A to C) of donor 21743 (week 2) RNA (see also panel A). Lane 9 shows that the endpoint variant from dilution B was the same as one of the endpoint variants from dilution C (10−3).