Because tier 1 and tier 2/3 Env trimers have open and closed conformations, respectively, and CD4 opens the closed conformation, we conclude that SERINC5 selectively dissociates Env trimers with an open conformation to restrict HIV-1 replication.<b>IMPORTANCE</b> Restriction factors provide the first line of defense against retrovirus infection by posing several blocks to the viral replication cycle.