CD4 and infection: Features common to all three of these species include an extremely low prevalence of CD4+ cells bearing CCR5 (the principle co-receptor for SIV) (Paiardini et al., 2011; Pandrea et al., 2007a), a large population of T-cells that are CD4–, but may be able to carry out functions borne normally by CD4+ cells (Beaumier et al., 2009; Milush et al., 2011; Murayama et al., 1999; Vinton et al., 2011), limited vertical transmission of SIV and suppression of immune activation after the acute stage of infection (Bosinger et al., 2009; Kornfeld et al., 2005; Onanga et al., 2002).