Different IFITM members exhibit various functions. IFITM5 is specifically expressed in bone cells, but could not be induced by IFN stimulation [3, 19] . IFITM5 is involved in bone formation and considered as a bone-specific modulator of mineralization. The vertebrate IFITM5 genes form an independent clade in IFITM family ( Fig. 2A) . Neither gene duplication nor positive selection was identified in IFITM5 sub-family ( Fig. 6B and Table 3 ), implying the functional conservation of IFITM5 in vertebrate evolution. Interestingly, no IFITM5 gene was found in the genomic data of two primate species, gorilla and marmoset. Gorilla has a close phylogenetic relationship with human and chimpanzee. In human and chimpanzee, IFITM5 is located upstream of the IFITM1-IFITM2-IFITM3 gene-cluster in chromosome 11. IFITM1, IFITM2 and IFITM3 in gorilla are also located in chromosome 11, and form a similar gene-cluster to those of human and chimpanzee (Fig. 4) . Therefore, gorilla is also presumed to have an IFITM5 gene upstream of the IFITM1-IFITM2-IFITM3 gene-cluster. In fact, we found a region with incomplete sequencing at the corresponding locus of human or chimpanzee IFITM5 genes in chromosome 11 of gorilla, explaining why we could not identify IFITM5 gene locus in current gorilla genome dataset. In contrast, marmoset has special IFITM gene organization and experienced rapid gene expansion, thus forming unique species-specific gene cluster (Fig. 3) . Considering that marmoset is one of the smallest monkeys in the world, as well as that IFITM5 plays a crucial role in bone formation [19, 36] , we inferred that marmoset most likely lost its IFITM5 during the long period of evolution. Nevertheless, whether the loss of IFITM5 gene contributes to the tiny size of the marmoset needs to be determined by future studies.