BST2 and infection: None of the SIV Nef proteins are able to counteract human tetherin, and this specificity maps to five amino acids (G/DDIWK) in the cytoplasmic domain of simian tetherin that are missing from the human orthologue [16,67–69], leading to speculation that prehistoric infection with viruses encoding Nef-like tetherin antagonists may have selected this deletion.