All participants will have a tetanus booster shot. Non-infected twins will undergo a procedure called apheresis to collect white blood cells. For this procedure, whole blood is collected through a needle in an arm vein, similar to donating blood. The blood is separated it into its components by centrifugation (spinning), the white cells are removed, and the rest of the blood is returned to the body, either through the same needle or through another needle in the other arm. The harvested white cells will be grown in culture for approximately 10 days to 2 weeks to increase their numbers up to 1000-fold. A gene called NeoR, which is derived from bacteria, will be inserted into the cells, and these gene-marked cells will be infused into the HIV-infected twin.