This is quite interesting. There is a certain very small percentage of the world's population that is immune to the HIV virus. These people's T-cells have the CD4 receptor, but not the CCR5 receptor. The HIV virus needs both these receptors to successfully affect the host's cells. The absence of the CCR5 receptor still allows the person's T cells to function normally, so if this works, it can pave the way to creating offspring who are immune to HIV