The V3 region WEAU did not span enough of the loop for prediction, and transmitted virus in WEAU was a R5/X4 virus [11], Thus, like the immune escape forms, single base changes that give rise to predicted CXCR4-using variants are likely to be present from early in infection, and could be available for rapid expansion when selection pressure arises [25]. The gene discussed is CXCR4; the disease is infection.