Bone-forming osteoblasts are derived from mesenchymal cells in bone marrow, and phenotypes present during osteoblastogenesis are tentatively divided into three phases: osteoprogenitor, preosteoblast, and mature osteoblast. Each of these phenotypes is characterized by specific marker genes. We recently analyzed comprehensive gene expression of osteoblast-like MC3T3-E1 cells cultured on various surfaces and found a novel gene, Ifitm5, involved in osteoblast maturation or new bone formation. This gene encodes a transmembrane protein expressed only in a late stage of osteoblast maturation. The aim of our study is to characterize this novel gene and to elucidate its function in the maturation of osteoblasts and the formation of new bone tissue. We have therefore produced Ifitm5 knockout mice and are analyzing the phenotypes of these mice, the histomorphology of their bones, and the gene and protein expression in their bone cells. We also screen the proteins interacting with the Ifitm5 product and transcription factors to regulate the expression of this gene. We hope to use this gene in the therapeutic treatment of people with bone-related diseases.