The largest scale of gene expansion of IR-IFITM genes was observed among the primates in a complex pattern that includes both lineage-and species-specific gene duplication events. The species-specific pattern mainly seems to have occurred in macaque and marmoset, giving rise to seven IFITM2&3-like genes in macaque and 29 IR-IFITM genes in marmoset (Fig. 3) . Large scale of duplication and pseudogenization events suggest that the IR-IFITM clade evolved under birth-and-death model [41, 42] . Interestingly, however, marmoset does not possess any one of IFITM1, IFITM2 or IFITM3, and macaque possesses IFITM1 but not IFITM2 and IFITM3. Why marmoset and macaque evolved so many IR-IFITM genes remains unclear and should be explored in more detail in future studies.