HIV mutations that help the virus bypass the host’s immune system often fall within gag. This gene is a favorite target for mutations because, for some reason, Gag epitopes that are presented by HLA B alleles create the most productive immune response. But Gag is also critical for viral replication. The infected individual doesn’t benefit from replication-dampening gag mutations, as the virus still escapes from cytotoxic T cells.

Goepfert and colleagues now find, however, that the resulting decreased replication may benefit the patient’s sexual partner. Their study of 114 couples found that individuals who were infected by partners with certain HLA B alleles had much lower viral levels when more than five gag mutations were present. Viral levels were lowest in newly infected individuals who did not have the same type of HLA B alleles as their partners, perhaps because the recipients’ T cells efficiently targeted non-Gag epitopes of the mutated virus.