However, because chimpanzee APOBEC3D has stronger antiviral activity compared to human APOBEC3D ([25] and S3 Fig) and SIVsmm Vif is not able to fully antagonize chimpanzee APOBEC3D (Fig 2A), it is possible that this host protein may be responsible for the reduced infection of the virus with SIVsmm vif in primary chimpanzee cells (Fig 3). Here, APOBEC3D is linked to infection.