As the factors responsible for differences in disease progression after infection with SIVmacΔnef have not yet been determined fully, the aim of this study was to characterize immunological parameters as well as host factors that contributed to differential disease outcome after experimental infection of rhesus monkeys with the SIVmac239ΔNU strain that carries a 513 bp deletion in nef (Gundlach et al., 1997). The gene discussed is S100B; the disease is infection.