IFITM1 (Interferon inducible transmembrane protein 1) is a member of the IFN-inducible transmembrane protein family. It is an essential mediator of interferon-gamma-induced antiproliferation and plays a role in the control of cell growth. IFITM1 is upregulated in several tumor types and may be useful as a tumor biomarker. Both mouse IFITM1 and IFITM3 are expressed on the cell surfaces of primordial germ cells in a developmentally-regulated manner. IFITM1 activity is required for primordial germ cell transit, and IFITM1 acts as a repulsive molecule by repelling non-IFITM1-expressing primordial germ cells from the mesoderm into the endoderm.