At present treatment for HIV infection involves using a combination of drugs. One type of drug inhibits nucleic acid replication, for example, when AZT is incorporated into DNA by reverse trasncriptase, no additional nucleotides can be added. A second class of drugs inhibits a protease involved in cleaving large polyproteins into functional segments. When the protease is inhibited, virus particles cannot be produced Use of a combination of drugs directed at different targets reduces the likelihood of the development of resistance by mutation. Research is underway to develop new treatments for HIV. Chemicals called chemokines appear to inhibit HIV infection by binding to and blocking the CXCR4 and CCR5 co receptors on the host cells. A search is in progress to apply such chemicals in HIV treatment,Some individuals who are HIV positive, but did not develop AIDS have a mutation in the CCR5 receptor that makes it defective. Research is in progress to determine whether disabling the CCR5 receptor might beeffective in controlling AIDS in infected patients.An antiviral factor called CAF is found in CD8 cells.Research is in progress to determine if this substance could be used to block replication of HIV. There is currently no approved vaccine against HIV infection. One problem is that HIVsurface proteins such as gp120 mutate at a high rate.Scientist may be able to construct a vaccine using an HIV with a defective form of the viral gene nef. Viruses with the defective nef gene seem to have reduced reproductive capability, allowing the immune system to keep the virus in check.