Likewise, similar to the rodent sub-clade, primate IR-IFITM genes also form different clusters, which contain orthologous IFITM genes from different species, indicating that most IR-IFITM members diverged prior to species separation. Furthermore, some IR-IFITM genes from same species cluster together and form species-specific sub-clusters, indicating that the IR-IFITM subfamily experienced a rapid expansion through gene duplications after the divergence of primates. Interestingly, a species-specific cluster was formed by the 25 IR-IFITM genes from marmoset, suggesting a rapid expansion of IR-IFITM genes by gene duplication. This speculation is supported by the four pairs of marmoset IFITM genes (IFITM-like8 and IFITM-like24, IFITM-like10 and IFITM-like29, IFITM-like14 and IFITM-like26, and IFITM-like11 and IFITM-like27) that exhibit very close genetic relationships, possibly indicating relatively recent gene duplication events. Additionally, the two genes in each pair are located in different chromosomes (Fig. 4B ), indicating their origination by segmental duplication rather than tandem duplication.