Note: this site last updated in 2006
An article from "The Molecules of HIV" (c) Dan Stowell
One of the earliest facts that was determined about how HIV works was that it binds to the CD4 protein which protrudes from many cells.
Later on, it was realised that other proteins are necessary for HIV to get into a cell. These are the "co-receptors".
Other strains of HIV require the CXCR4 co-receptor.
There have been reports of other possible co-receptors, but as far as we know those two are the important ones.