tropism – the type of coreceptor used by HIV in order to attach to and then infect a cell. If HIV uses the CCR5 coreceptor on the surface of the a CD4 cell it is called R5-tropic. If it uses the CXCR4 co receptor it is called R4-tropic). Early HIV infection is usually R5-tropic but over time, especially in late disease (if CD4 counts drop to less than 50 cells/mm3) the virus shifts to being X4-tropic.  Mixed tropic refers to a having some viruses that use R5 and some that use X4.