CD4 and infection: Studies of simianimmunodeficiency virus (SIV) infections of non-human primates suggest thatdifferences in the expression level of CCR5 on target CD4+ cells mayunderlie the difference between the non-pathogenic infection of natural hosts, suchas African green monkeys and sooty mangabeys, and the pathogenic infection ofnon-natural hosts, such as rhesus macaques [1]–[3].