In addition, infection with mutant forms of SIVmac239 provided insights into the relevance of specific Nef functions, such as downmodulation of the CD4 receptor or MHC class I molecules, for viral immune evasion and replication fitness (Brenner et al., 2006; Münch et al., 2001; Schindler et al., 2004; Swigut et al., 2004). The gene discussed is CD4; the disease is infection.