Note: this site last updated in 2006
An article from "The Molecules of HIV" (c) Dan Stowell
Scientists studying HIV-1 discovered that different forms of HIV use different coreceptors. The most commonly-transmitted strains of HIV use the CCR5 coreceptor, and the most common strains that develop within an AIDS patient use CXCR4. Therefore, many researchers refer to strains of HIV as follows:
R5 = "HIV strains which use the CCR5 coreceptor"
X4 = "HIV strains which use the CXCR4 coreceptor"
There are, by the way, "R5X4" strains of HIV which can use either of these receptors.