The Red Queen hypothesis presumes that the antagonistic coevolutionary dynamics of host-virus systems can generate selection Table 3 . Site-specific tests for positive selection on different IFITM sub-families. for accelerated evolution of host antiviral restriction factors, just like the observations on the primate antiviral restriction factors APOBEC3G, Tetherin, and SAMHD1 [21] [22] [23] [24] [25] [26] [27] [28] [29] . We detected positive selection acting on marmoset IR-IFITM genes (Table 3) . Macaque and marmoset are susceptible to infection by many contemporary viruses and are often used as suitable non-human primate models for viral infectious disease studies [9, [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] . Cell lines (e.g. kidney cells) from marmoset can be infected by most primate viruses, such as flaviviridae family of viruses (GBV-B), lassa virus, peste des petitis ruminants (PPR) virus and so on [59, 60, 61] . These imply that marmoset and macaque might be able to be infected by the ancestors of contemporary primate viruses and/or some unknown viruses during early evolution of primates. Additionally, relative to pig IFITM3 that has been demonstrated to have an antiviral activity [40] , IFITM1, IFITM2 and IFITM3 genes from macaque and marmoset have higher similarity to human IFITM1, IFITM2 and IFITM3 in sequence and domain organization. These imply that although lacking experimental support, the IR-IFITM genes from macaque and marmoset might have similar antiviral activity to human IFITM1, IFITM2 and IFITM3. Therefore, the rapid expansion of IR-IFITM genes might be ascribed to the infection of marmosets by viruses.