Note: this site last updated in 2006
An article from "The Molecules of HIV" (c) Dan Stowell
CXCR4 (aka "fusin") and R5">CCR5 are both chemokine receptors. That is, they are proteins normally embedded in the membrane of a cell, and responds to the presence of chemokines">chemokines outside the cell to trigger some kind of response within the cell.
HIV-1 is able to use either CXCR4 or R5">CCR5 as a co-receptor (CD4 being the main receptor) to facilitate binding and entry into T cells. There are other possible co-receptors, but these are most commonly used.
CXCR4 was the first HIV coreceptor to be discovered, and it's one of the most important ones.